Extended wavelength strained layer lasers having nitrogen disposed therein

ABSTRACT

Several methods are used in novel ways with newly identified and viable parameters to decrease the peak transition energies of the pseudomorphic InGaAs/GaAs heterostructures. These techniques, taken separately or in combination, suffice to permit operation of light emitting devices at wavelengths of 1.3 μm or greater of light-emitting electro-optic devices. These methods or techniques, by example, include: (1) utilizing new superlattice structures having high In concentrations in the active region, (2) utilizing strain compensation to increase the usable layer thickness for quantum wells with appropriately high In concentrations, (3) utilizing appropriately small amounts of nitrogen (N) in the pseudomorphic InGaAsN/GaAs laser structure, and (4): sue of nominal (111) oriented substrates to increase the usable layer thickness for quantum wells with appropriately high In concentrations. In all of the above techniques, gain offset may be utilized in VCSELs to detune the emission energy lower than the peak transition energy, by about 25 meV or even more, via appropriate DBR spacing. Gain offset may also be utilized in some forms of in-plane lasers. Increased temperature may also be used to decrease peak transition energy (and therefore the emission energy) by about 50 meV/100° C. All these techniques are furthermore applicable to other material systems, for example, extending the emission wavelength for laser diodes grown on InP substrates. Additionally, structures which utilize the above techniques are discussed.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application makes reference to the following co-pending U.S.patent applications. The first application is U.S. App. Ser. No.08/547,165, entitled “Conductive Element with Lateral OxidationBarrier,” filed Dec. 18, 1995. The second application is U.S. App. Ser.No. 08/659,942, entitled “Light Emitting Device Having an ElectricalContact Through a Layer containing Oxidized Material,” filed Jun. 7,1996. The third application is U.S. App. Ser. No. 08/686,489 entitled“Lens Comprising at Least One Oxidized Layer and Method for FormingSame,” filed Jul. 25, 1996. The fourth application is U.S. App. Ser. No.08/699,697 entitled “Aperture comprising an Oxidized Region and aSemiconductor Material,” filed Aug. 19, 1996. These applications arehereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to semiconductor lightsources such as LEDs and VCSELs, and more particularly to a strainedlayer semiconductor laser having an emission wavelength of at least 1.3μm.

[0004] 2. Description of the Prior Art

[0005] Vertical-Cavity Surface-Emitting Lasers (VCSELs), Edge EmittingLasers (EELs) or Light Emitting Diodes (LEDs) are becoming increasinglyimportant for a wide variety of applications including opticalinterconnection of integrated circuits, optical computing systems,optical recording and readout systems, and telecommunications.Vertically emitting devices have many advantages over edge-emittingdevices, including the possibility for wafer scale fabrication andtesting, and the possibility of forming two-dimensional arrays of thevertically emitting devices. The circular nature of the light outputbeams from these devices also make them ideally suited for coupling intooptical fibers as in optical interconnects or other optical systems forintegrated circuits and other applications.

[0006] For high-speed optical fiber communications, laser or LEDemission wavelengths in the 1.3 μm through 1.55 μm region are desired.Standard silica fiber has zero dispersion near 1.3 μm and has a minimumloss near 1.55 μm. The need for semiconductor lasers emitting in thiswavelength region has spawned worldwide development of such lasers.Group III-V semiconductors which emit light in the 1.3 through 1.55 μmregion have lattice constants which are more closely matched to InP thanto other binary III-V semiconductor substrates, for example, GaAs. Thus,essentially all commercial emitting lasers emitting at 1.3 through 1.55μm are grown on InP substrates. These lasers are edge-emitting laserswhich, unlike VCSELs, do not require high-reflectivity Distributed BraggReflectors (DBRs) to form their optical cavities.

[0007] Unfortunately, it has proven difficult to produce effective DBRson InP substrates. The available materials which lattice match InP haveproduced mirrors which are extremely thick and lossy and have thus notresulted in efficient VCSELs.

[0008] VCSELs or Surface Emitting Lasers SELs whose current flow iscontrolled by lateral oxidation processes have show the bestperformances of any VCSELs in terms of low threshold current, highefficiency, and high speed. All such “oxide VCSELs” have been fabricatedusing AlAs or AlGaAs layers which were grown on GaAs substrates andlater oxidized. Thus, one would want to utilize a VCSEL structure suchas is disclosed in U.S. Pat. No. 5,493,577, by Choquette et al. ThisVCSEL has the advantages of: (1) reduced mode hopping; (2) beingtemperature stable, and (3) testable in a modified silicon wafer tester.Unfortunately, this VCSEL structure will have an emission wavelengthbetween 600 and 1,000 nm and thus falls short of the desired 1.3 μmemission wavelength. Due to the availability of well-behaved oxidizablematerials which may be grown on GaAs substrates and the straightforwardcapability of producing efficient high-reflectivity DBRs on GaAssubstrates, when manufacturing VCSELs it is highly desirable to growthem on GaAs substrates.

[0009] The salient components of a VCSEL typically include two DBRs, andbetween them, a spacer which contains an active region having a lengthemitting material. The DBRs and active region form an optical cavitycharacterized by a cavity resonance at a resonant wavelengthcorresponding to a resonant photon energy. It has become a practice inthe operation of VCSELs to detune the optical cavity to energies atabout 25 meV lower than the peak transition energy by appropriate DBRspacing. Such “gain offset” is used to advantage in reducing temperaturesensitivity. This produces an emission wavelength which is appreciablylonger than the peak transition wavelength. This practice, whileinadequate in itself for increasing emission wavelength to 1.3 μm frommaterial grown pseudomorphically on GaAs substrates, does measurablyincrease emission wavelength. Even if this technique was incorporatedwith the teachings of the prior art, one would fall short of the desired1.3 μm emission wavelength.

[0010] While epitaxial growth of slightly-lattice-mismatched materialsis undertaken routinely, materials which emit in the 1.3 μm through 1.55μm region have lattice constants sufficiently removed from that of GaAsto make pseudomorphic epitaxial growth problematic. In this context,“pseudomorphic” means having a sufficiently low density of misfitdislocations such that lasers may be produced which have reasonably longlifetimes. The problems have been sufficiently great to causeresearchers to abandon such efforts and resort to less desirable hybridapproaches to producing 1.3 μm through 1.55 μm VCSELs.

[0011] Thus, the production of VCSELs emitting at 1.3 through 1.55 μmwavelengths has been inhibited by either of two problems. The problemsresult from the fact that VCSELs require laser-quality active materialsand high-reflectivity DBR mirrors. These two problems are:

[0012] (1) when InP substrates are used, growth of the light emittingactive material is straightforward, but production of efficient DBRs isdifficult and has not been effective; and

[0013] (2) when GaAs substrates are used, DBR production isstraightforward, but efforts lo glow laser-quality active material havebeen unsuccessful.

[0014] The following is a summary of the prior approaches which arerelevant to the problem of producing 1.3 though 1.55 μm VCSELs.

[0015] A 1.3 μm edge-emitting laser grown on a GaAs substrate wasreported by Omura et al., in an article entitled “Low Threshold Current1.3 μm GaInAsP Lasers Grown on GaAs Substrates,” Electronics Letters,vol. 25, pp. 1718-1719, Dec. 7, 1989. The structure comprises a layerhaving a high density of misfit dislocations, on top of which were grownthick layers of materials having lattice constants close to that of InP.Such lasers exhibit very poor reliability due to the misfitdislocations. Furthermore, this structure does not readily lend itselfto integration with DBR mirrors.

[0016] The use of a layer having high-density misfit dislocations wasalso reported by Melman et al., in an article entitled “InGaAs/GaAsStrained Quantum Wells with a 1.3 μm Band Edge at Room Temperature,”Applied Physics Letters, vol. 55, pp. 1436-1438, Oct. 2, 1989. Thearticle states that pseudomorphic, i.e., nearly misfit dislocation free,growth of 1.3 μm emitting material is not possible with GaAs barriers,i.e., GaAs substrates. This conclusion prompted the approach toincorporate a layer having a high density of misfit dislocations.

[0017] A 1.1 μm emitting laser is reported in Waters et al., in anarticle entitled “Viable Strained Layer Laser at λ=1100 nm.” Journal ofApplied Physics, vol. 67, pp. 1132-1134, Jan. 15, 1990. The laserutilized a single quantum well comprising In_(0.45)Ga_(0.55)As strainedsemiconductor material which has a greater thickness than its predictedcritical thickness. Reliability tests are presented for 4000 hours oftesting. To our knowledge, these are the longest-wavelength lasersproduced on GaAs substrates which have survived such testing. In thisarticle, even Waters recognizes the difficulty of creating a reliabledevice having an active region over the respective CT for thesemiconductor material in the active region.

[0018] A strained quantum well emitting at 1.3 μm is reported by Roanand Chang in an article entitled “Long-Wavelength (1.3 μm) Luminescencein InGaAs Strained Quantum-Well Structures grown on GaAs.” AppliedPhysics Letters, vol. 59, pp. 2688-2690, Nov. 18, 1991. The quantum wellwas a short-period superlattice comprising alternating monolayers ofInAs and GaAs. However, the quantum well had a thickness well above(1.78 times) the critical thickness, above which densities of misfitdislocations exist. Thus, the structure is not viable for long-livedlasers and no lasers were produced from such a structure.

[0019] A compromise between GaAs and InP substrates is reported bySahoji et al., in an article entitled “Fabrication ofIn_(0.25)Ga_(0.75)As/InGaAsP Strained SQW Lasers on In_(0.05)Ga_(0.95)AsTernary Substrate,” IEEE Photonic Technology Letters, vol. 6, no. 10,pp. 1170-1172, Oct. 10, 1994. An In_(0.05)Ga_(0.95)As ternary substratewas utilized which has a lattice constant intermediate between those ofGaAs and InP. The In concentration of the substrate was 5% of the groupIII material (2.5% of the total material) and the laser emitted at 1.3μm. The authors indicate that 1.3 μm lasers will require an InGaAssubstrate having about 25% or more in for the group-III material.Ternary substrates are unlikely to approach the availability, size andprice of binary substrates such as GaAs.

[0020] James Coleman, in his book entitled “Quantum Well Lasers,” editedby Peter Zory, London, Academic Press, pp. 372-413, 1993, discusses theconcept of critical thickness in strained layers lasers which utilizeIn_(y)Ga_(1-y)As. As may be seen in FIG. 4 of this reference, as thecomposition of In increases, i.e., y approaches 0.5, the criticalthickness drops dramatically. Turning now to FIG. 10 of this reference,it may be seen that Coleman has demonstrated that as the Inconcentration increases, the peak transition wavelength increases in asub-linear fashion. As the In concentration approaches 0.5 the peaktransition wavelength approaches about 1.20 μm. If one was toextrapolate information from this graph for In concentrations greaterthan or equal to 0.5, one would come to the clear conclusion that a peaktransition wavelength of 1.3 μm is not obtainable while maintaining theIn_(y)Ga_(1-y)As layer within the critical thickness. Thus, whileColeman does provide a valuable teaching, he is unable to reach a 1.3 μmpeak transition wavelength.

[0021] The issue of strain compensation to increase the number ofstrained quantum wells which may be grown without misfit dislocations isfrequently used in the art and is described by Zhang and Ovtchinnikov inan article entitled “Strain-compensated InGaAs/GaAsP/GaInAsP/GaInPQuantum Well Lasers (λ˜0.98 μm) Grown by Gas-Source molecular BeamEpitaxy.” Applied Physics Letters, vol. 62, pp. 1644-1646, 1993. Thereader is also referred to U.S. Pat. No. 5,381,434 by Bhat and Zah.

[0022] The advantages of incorporating strain into the active region ofa semiconductor laser were described by Yablonovitch in U.S. Pat. No.4,804,639. Yablonovitch discloses active regions of In_(y)Ga_(1-y)Asgrown on GaAs substrates, typically with y˜0.5, and having a thicknesspreferably less than 100 Å. He suggests the possibility of “the additionof counter-strain layers of GaP on either side of the active strainedlayer,” but does not pursue this possibility. He goes on to performnumerical evaluations based on “an assumed set of numerical coefficientswhich are thought to be representative of a quaternary semiconductorwith a band edge near the 1.5 μm wavelength.” The material is furtherassumed to have a strain of 3.7% and a thickness of 100 Å which wasthought to be “probably the maximum permissible thickness for such ahigh strain.” This strain for y=0.5 is calculated to be less than ˜40 Å.Thus although a >1.3 μm emitting laser utilizing strained InGaAs on GaAsis indirectly suggested, no actual structure is specified and theparameters are not realistic.

[0023] In U.S. Pat. No. 5,060,030, Hoke describes improvements inelectron mobility and electron saturation for use inhigh-electron-mobility transistors (HEMTs). He describes the use ofstrain compensation to increase the thickness or In concentration “byapproximately a factor of two” in a strained InGaAs layer grown on GaAs.

[0024] A strain-compensated heterostructure laser diode is described byBuchan et al. in U.S. Pat. No. 5,373,166. Buchan describes gradedstructures in the compressive and tensile strained quaternary layerswith the strain magnitudes less than 1%. The thicknesses of the layersdescribed are less than their conventional critical thicknesses.

[0025] Vawter et al., in an article entitled “Useful DesignRelationships for the Engineering of Thermodynamically StableStrained-layer Structures,” Journal of Applied Physics, vol. 65, pp.4769-4773, 1989, describes approaches for engineering dislocation-freestrained-layer structures. The article includes a methodology forcalculating the “critical thickness” of structures comprising layers ofdiffering lattice constants. The methodology is based upon a “reducedeffective strain” which is the sum of the strain-thickness products ofall the layers divided by the total thickness of the layers. Based uponthis “reduced effective strain,” the “critical thickness” for thestructures is their calculated from a critical thickness criterion,e.g., that introduced by Matthews and Blakeslee.

[0026] Asahi et. al., in an article entitled “New III-V CompoundSemiconductors TlInGaP . . . ” Japanese Journal of Applied Physics,describes the inclusion of the group-III element thallium (Tl) in III-Vsemiconductors for long-wavelength emission. Most of the discussionfocuses on lasers emitting at wavelengths greater than 2 μm on InPsubstrates, but it is stated that TlGaP lattice-matched to GaAssubstrate has a bandgap emission of about 1.24 μm. The extreme toxicityand hazardous nature of Tl, even after epitaxial growth is performed,makes it undesirable as a manufacturing material.

[0027] Very recently, it has been shown that adding nitrogen to InGaAs,actually decreases the peak transition energy and thereby increases thepeak transition wavelength as described by Kondow et al., in all articleentitled “GaInNAs: A Novel Material for Long-Wavelength-Range LaserDiodes with Excellent High-Temperature Performance,” Jpn. J. Appl.Phys., vol. 35. pp. 1273-1275, February 1996. The report suggests thatit is difficult to grow high quality InGaAsN with very much N. A roomtemperature photo-luminescence spectrum of a 70 Å thickIn_(y)Ga_(1-y)As_(1-y)N_(v)/GaAs quantum well showed significantbroadening even with onl yless than 1% N concentration for the group Vsemiconductor element. This corresponds to a value of v being less than0.01. The peak transition wavelength of this semiconductor was 1.23 μm.In the report, the authors state that their plan is to reach a 1.3 μmdevice by increasing the N concentration to 1% while maintaining a 30%In concentration.

[0028] A hybrid approach to address the dual problem described earlierhas been reported by Margalit et al., in an article entitled “LaterallyOxidized Long Wavelength CW Vertical-Cavity Lasers.” Applied PhysicsLetters, vol. 69, pp. 471-472, Jul. 22, 1996. In this work, two DBRs aregrown on two separate GaAs substrates, while the active material isgrown on a third substrate which comprises InP. The active materialcomprises seven compressively strained InGaAsP quantum wells clad by 300nm of InP on each side. To assemble these materials, two processes areperformed, each including the thermal fusion of two wafers and removalof one substrate. Then the resulting structure is processed by standardVCSEL processing methods.

[0029] Since VCSELs are presently the subject of intense research anddevelopment, a great deal of results and advancements are publishedperiodically. The following is a list of documents which are relevant tothe problem of extending emission wavelengths of semiconductor lasers orof producing 1.3 μm through 1.55 μm VCSELs.

[0030] Fisher et al., “Pulsed Electrical Operation of 1.5 μmVertical-Cavity Surface Emitting Lasers,” IEEE Photonics TechnologyLetters, Vol. 7, No. 6, pp. 608-609, Jun. 6, 1995.

[0031] Uchiyama et al., “Low Threshold Room Temperature Continuous WaveOperation of 1.3 μm GaInAsP/InP Strained Layer Multiquantum Well SurfaceEmitting Laser,” Electronics Letters, vol. 32, no. 11, pp. 1011-1013,May 23, 1996.

[0032] Hasenberg, “Linear Optical Properties of Quantum Wells Composedof All-Binary InAs/GaAs Short-Period Strained-Layer Superlattices,”Applied Physics Letters., vol. 58, no. 9, pp. 937-939, Mar. 4, 1991.

[0033] Fukunaga et al., “Reliable Operation of Strain-Compensated 1.06μm InGaAs/InGaAsP/GaAs Single Quantum Well Lasers,” Applied PhysicsLetters., vol. 69, no. 2, pp. 248-250, Jul. 8, 1996.

[0034] Kondow et al., “Gas-Source Molecular Beam Epitaxy of GaN_(x)As_(1-x) Using a N Radical as the N Source,” Jpn. J. Appl. Phys., vol.33, pp. 1056-1058, Aug. 1, 1994.

[0035] Shimomura et al., “Improved Reflectivity of AlPSb/GaPSb BraggReflector for 1.55 μm Wavelength,” Electronics Letters, vol. 30, no. 25,pp. 2138-2139, Dec. 8, 1994.

[0036] Blum et al., “Wet Thermal Oxidation of AlAsSb Lattice Matched toInP for Optoelectronic Applications,” Applied Physics Letters., vol. 68,no. 22, pp. 3129-3131, May 27, 1996.

[0037] Mirin, R. P., “1.3 μm Photoluminescence From InGaAs quantum dotson GaAs,” Applied Physics Letter., vol. 67, no. 25, pp. 3795-3797, Dec.18, 1995.

[0038] Thus, although the prior art therefore describes a variety oftechniques useful in forming long-wavelength lasers on GaAs substrates,it fails to provide any specific example of a viable such structure, nordoes it provide any range of parameters within which viable suchstructures may be fabricated, nor does it teach the construction of aviable such structure. Some references suggest the possibility of 1.3 μmlasers on GaAs substrates, but provide unrealistic parameters and areseveral years old or more.

SUMMARY OF THE INVENTION

[0039] It is therefore an object of the present invention to provide anactive region having a quantum well structure which may be utilized inlasers grown on GaAs substrates and which will provide an emissionwavelength of at least 1.3 μm. Extensive work on novel strainedInGaAs/GaAs heterostructures have led to an unexpected conclusion whichcontradicts the conclusions reached in the prior art. It has been foundthat use of high-indium-content structures permits emission wavelengthsof at least 1.3 μm using active layers grown on GaAs substrates which donot exceed their critical thickness. Other techniques allowpseudomorphic growth of active layers above their nominal criticalthickness. These techniques, carefully applied to newly-identifiedparameter spaces, allow the unexpected result of pseudomorphicstructures grown on GaAs substrates which emit at 1.3 μm and longerwavelengths. Parameter spaces are defined for viable structures grown onGaAs substrates and emitting at 1.3 μm or longer. Specific examples ofthese viable structures are provided in the detailed description, below.

[0040] Several methods are used in novel ways with newly identified andviable parameters to decrease the peak transition energies of thepseudomorphic InGaAs/GaAs heterostructures. These techniques, takenseparately or in combination, suffice to permit operation at wavelengthsof 1.3 μm or greater of light-emitting electro-optic devices. Thesemethods or techniques, by example, include: (1) utilizing newsuperlattice structures having high In concentrations in the activeregion, (2) utilizing strain compensation to increase the usable layerthickness for quantum wells with appropriately high In concentrations,(3) utilizing appropriately small amounts of nitrogen (N) in thepseudomorphic InGaAsN/GaAs laser structure, and (4) sue of nominal (111)oriented substrates to increase the usable layer thickness for quantumwells with appropriately high In concentrations. In all of the abovetechniques, gain offset may be utilized in VCSELs to detune the emissionenergy lower than the peak transition energy, by about 25 meV or evenmore, via appropriate DBR spacing. Gain offset may also be utilized insome forms of in-plane lasers. Increased temperature may also be used todecrease peak transition energy (and therefore the emission energy) byabout 50 meV/100° C. All these techniques are furthermore applicable toother material systems, for example, extending the emission wavelengthfor laser diodes grown on InP substrates.

[0041] It is a further object to provide various techniques which may beutilized in combination with high In concentrations to reduce the peaktransition energy of a device, having a GaAs substrate, to allow for anemission wavelength of 1.3 μm or greater.

[0042] It is yet another object to provide a pseudomorphic superlatticestructure on a GaAs substrate which reduces the peak transition energysufficiently to allow for an emission wavelength of 1.3 μm or greater.

[0043] According to one broad aspect of the present invention, there isprovided a light emitting device having at least a substrate and anactive region, the light emitting device comprising the substrate havinga substrate lattice constant between 5.63 Å and 5.67 Å; the activeregion comprising at least one pseudomorphic light emitting active layerdisposed above the substrate, the active layer comprising at least In,Ga, As and N, the active layer having a thickness equal to or less thanan respective CT, where:

CT=(0.4374/f)[In(CT/4)+1],

[0044] where f is an average lattice mismatch of the active layernormalized to a lattice constant of 5.65 Å, the active layer having anaverage sum of In and Sb concentrations in the active layer at 16.5% orgreater of a semiconductor material in the active layer and the nitrogencontent less than 1% of a group V semiconductor material in the activeregion; and wherein the light emitting device has an emission wavelengthof at least 1.3 μm.

[0045] According to another broad aspect of the invention, there isprovided a light emitting device having at least at substrate and anactive region, the light emitting device comprising: the substratecomprising having a substrate lattice constant between 5.63 Å and 5.67♦; the active region comprising at least one pseudomorphic lightemitting active layer disposed above the substrate, the active layercomprising at least In, Ga and As, the active layer having a thicknessequal to or less than 1.25 times a respective CT, where:

CT=(0.4374/f)[In(CT/4)+1],

[0046] where f is an average lattice mismatch of the active layernormalized to a lattice constant of 5.65 Å; wherein the active layercomprises at least two strained layers, and a third layer disposedbetween the two strained layers, forming a superlattice where an averagesum of In and Sb concentrations in the superlattice is 25% or greater ofa semiconductor material in the active layer; and wherein the lightemitting device has an emission wavelength of at least 1.3 μm.

[0047] According to another broad aspect of the invention, there isprovided a light emitting device having at least a substrate and anactive region, the light emitting device comprising: the substratecomprising having a substrate lattice constant between 5.63 Å and 5.67Å; the active region comprising at least one pseudomorphic lightemitting active layer disposed above the substrate, the active layercomprising at least In, Ga and As, the active layer having a thicknessequal to or less than 1.25 times a respective CT, where:

CT=(0.4374/f)[In(CT/4)+1],

[0048] where f is an average lattice mismatch of the active layernormalized to a lattice constant of 5.65 Å, wherein the active layercomprises at least two strained layers, and a third layer disposedbetween the two strained layers, forming a superlattice where an averagesum of In and Sb concentrations in the superlattice is greater than 25%of a semiconductor material in the active layer; a first conductivelayer having a first conductivity type, the first conductive layerdisposed in electrical communication with the active layer; a secondconductive layer having a second conductivity type, the secondconductive layer being disposed above the active layer and in electricalcommunication therewith; and electrical communication means forproviding electrical current to the active layer; and wherein the lightemitting device has an emission wavelength of at least 1.3 μm.

[0049] According to another broad aspect of the invention, there isprovided a light emitting device having at least a substrate and anactive region, the light emitting device comprising: the substratehaving a substrate lattice constant between 5.63 Å and 5.67 Å; a firststrained layer having a lattice constant smaller than the substratelattice constant and being disposed between the substrate and the activeregion; the active region comprising at least one pseudomorphic lightemitting active layer disposed above the substrate, the active layercomprising at least In, Ga and As, the active layer comprises at leasttwo strained layers, and a third layer disposed between the two strainedlayers, the active layer having a thickness equal to or less than 80 Å;and wherein the light emitting device has an emission wavelength of atleast 1.3 μm.

[0050] According to another broad aspect of the invention, there isprovided a light emitting device having at least a substrate and anactive region, the light emitting device comprising: the substratehaving a substrate lattice constant between 5.63 Å and 5.67 Å; a firststrained layer having a lattice constant smaller than a substratelattice constant and being disposed between the substrate and the activeregion; the active region comprising at least one pseudomorphic lightemitting active layer disposed above the substrate, the active layercomprising at least In, Ga and As; the active layer having aconcentration of In and Sb of 25% or greater of a semiconductor materialin the active layer, the active layer having a thickness greater than CTand less than 2.5 times CT for a given material, where:

CT=(0.4374/f)[In(CT/4)+1],

[0051] wherein f is an average lattice mismatch of the layer normalizedto a lattice constant of 5.65 Å; wherein the light emitting device hasan emission wavelength of at least 1.3 μm.

[0052] According to another broad aspect of the invention, there isprovided a light emitting device having at least a substrate and anactive region, the light emitting device comprising: the substratecomprising having a substrate lattice constant between 5.63 Å and 5.67Å; the active region comprising at least one pseudomorphic lightemitting active layer disposed above the substrate, the active layercomprising at least In, Ga, As and N, the active layer having athickness equal to or less than a respective CT, where:

CT=(0.4374/f)[In(CT/4)+1],

[0053] where f is an average lattice mismatch of the active layernormalized to a lattice constant of 5.65 Å; wherein the active layercomprises at least two strained layers, and a third layer disposedbetween the two strained layers, forming a superlattice having anitrogen content of at least 0.01% of a group V semiconductor materialin the active region; and wherein the light emitting device has anemission wavelength of at least 1.3 μm.

[0054] According to another broad aspect of the invention, there isprovided a light emitting device having at least a substrate and anactive region, the light emitting device comprising: the substratecomprising having a substrate lattice constant between 5.63 Å and 5.67Å; a first strained layer disposed between the substrate and the activeregion, the first strained layer having a first accumulated strain and afirst critical accumulated strain associated therewith, the firstaccumulated strain being less than the first critical accumulatedstrain; the active region comprising at least one pseudomorphic lightemitting active layer disposed above the substrate, the active layercomprising at least In, Ga, As and N, the active layer comprising atleast two strained layers, and a third layer disposed between the twostrained layers, forming a superlattice having a nitrogen content of atleast 0.01% of a group V semiconductor material in the active region,the active layer having a second accumulated strain and a secondcritical accumulated strain associated therewith, the algebraic sum ofthe first and second accumulated strains being less than the secondcritical accumulated strain; wherein the first and second criticalaccumulated strain for a given material equal a strain of the materialmultiplied by CT for a given material, where:

CT=(0.4374/f)[In(CT/4)+1],

[0055] where f is an average lattice mismatch of the material normalizedto a lattice constant of 5.65 Å; and wherein the light emitting devicehas an emission wavelength of at least 1.3 μm.

[0056] According to another broad aspect of the invention, there isprovided a light emitting device having it least a substrate and allactive region, the light emitting device comprising: the substratecomprising having a substrate lattice constant between 5.63 Å and 5.67Å; a first strained latter having a lattice constant smaller than aidsubstrate lattice constant and being

comprising at least In, Ga, As and N, the active layer comprises atleast two strained layers, and a third layer disposed between the twostrained layers, the active layer having a nitrogen concentration of atleast 0.01% of a group V semiconductor material in the active layer, theactive layer having a thickness equal to or less than 175 Å; and whereinthe light emitting device has an emission wavelength of at least 1.3 μm.

[0057] According to another broad aspect of the invention, there isprovided a light emitting device having at least a substrate and anactive region, the light emitting device comprising: the substratecomprising having a substrate lattice constant between 5.63 Å and 5.67Å; the substrate comprising having a substrate lattice constant between5.63 Å and 5.67 Å; a first strained layer having a lattice constantsmaller than aid substrate lattice constant and being disposed betweenthe substrate and the active region; the active region comprising atleast one pseudomorphic light emitting active layer disposed above thesubstrate, the active layer comprising at least In, Ga, As and N, theactive layer comprises at least two strained layers, and a third layerdisposed between the two strained layers, the active layer having anitrogen concentration of at least 0.01% of a group V semiconductormaterial in the active layer, the active layer having a thickness equalto or greater than CT for a given material, where:

CT=(0.4374/f)[In(CT/4)+1],

[0058] where f is an average lattice mismatch of the active layernormalized to a lattice constant of 5.65 Å; and wherein the lightemitting device has an emission wavelength of at least 1.3 μm.

[0059] According to another broad aspect of the invention, there isprovided a light emitting device having at least a substrate and anactive region, the light emitting device comprising: the substratecomprising) having a substrate lattice constant between 5.63 Å and 5.67Å; a first strained layer disposed between the substrate and the activeregion, the first strained layer having a first accumulated strain and afirst critical accumulated strain associated therewith, the firstaccumulated strain being less than the first critical accumulatedstrain; the active region comprising at least one pseudomorphic lightemitting active layer disposed above the substrate, the active layercomprising at least In, Ga, As and N, the active layer comprises atleast two strained layers, and a third layer disposed between the twostrained layers, the active layer having an average sum of In and Sbconcentrations in the superlattice at 33% or greater and the nitrogencontent of at least 0.01% of a group V semiconductor material in theactive region, the active layer having a second accumulated strain and asecond critical accumulated strain associated therewith, the algebraicsum of the first and second accumulated strain being less than thesecond critical accumulated strain; wherein the first and secondcritical accumulated strain for a given material equals a strain of thematerial multiplied by CT for a given material, where:

CT=(0.4374/f)[In(CF/4)+1],

[0060] where f is an average lattice mismatch of the active layernormalized to a lattice constant of 5.65 Å; and wherein the lightemitting device has an emission wavelength of at least 1.3 μm.

[0061] According to another broad aspect of the invention, there isprovided a light emitting device having at least a substrate and anactive region, the light emitting device comprising: the substratecomprising having a substrate lattice constant between 5.63 Å and 5.67Å; a first strained layer having a lattice constant smaller than thesubstrate lattice constant and being disposed between the substrate andthe active region; the active region comprising at least onepseudomorphic light emitting active layer disposed above the substrate,the active layer comprising at least In, and Ga, the active layercomprising at least two strained layers, and a third layer disposedbetween the two strained layers, the active layer having a secondaccumulated strain and a second critical accumulated strain associatedtherewith, the algebraic sum of the first and second accumulated strainsbeing less than the second critical accumulated strain; and wherein thelight emitting device has an emission wavelength of at least 1.3 μm.

[0062] According to another broad aspect of the invention, there isprovided a light emitting device having at least a substrate and anactive region, the light emitting device comprising: the substratecomprising having a substrate lattice constant between 5.63 Å and 5.67Å; a first strained layer disposed between the substrate and the activeregion, the first strained layer having a first accumulated strain and afirst critical accumulated strain associated therewith, the firstaccumulated strain being less than the first critical accumulatedstrain; the active region comprising at least one pseudomorphic lightemitting active layer disposed above the substrate, the active layercomprising at least In, Ga and As, the active layer comprises at leasttwo strained layers, and a third layer disposed between the two strainedlayers, forming a superlattice having an average sum of In and Sbconcentrations in the superlattice at 25% or greater of a semiconductormaterial in the active layer, the active layer having a secondaccumulated strain and a second critical accumulated strain associatedtherewith, the second accumulated strain being less than the secondcritical accumulated strain; wherein the first and second criticalaccumulated strain for a given material equals a strain of the materialmultiplied by CT for a given material, where:

CT=(0.4374/f)[In(CT/4)+1],

[0063] where f is an average lattice mismatch of the active layernormalized to a lattice constant of 5.65 Å; and wherein the lightemitting device has an emission wavelength of at least 1.3 μm.

[0064] According to another broad aspect of the invention, there isprovided a light emitting device having at least a substrate and anactive region) the light emitting device comprising: the substratecomprising having a substrate lattice constant between 5.63 Å and 5.67 Åand having a growth plane which has an orientation within 15° of (111);the active region comprising at least one pseudomorphic light emittingactive layer disposed above the substrate, the active layer comprisingat least In, Ga and As, the active layer having a thickness equal to orless than twice a respective CT, where:

CT=(0.4374/f)[In(CT/4)+1],

[0065] where f is an average lattice mismatch of the active layernormalized to a lattice constant of 5.65 Å; wherein the active layer hasan average sum of In and Sb concentrations of equal to or greater than25% or greater of a semiconductor material in the active layer; andwherein the light emitting device has an emission wavelength of at least1.3 μm.

[0066] Other objects and features of the present invention will beapparent from the following detained description of the preferredembodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

[0067] The invention will be described in conjunction with theaccompanying drawings, in which:

[0068]FIG. 1 is a graph of Peak Transition Energy v. Quantum WellThickness, at 300K, for several In concentrations of InGaAs strainedquantum wells on a GaAs substrate with GaAs barriers constructed inaccordance with a preferred embodiment of the invention;

[0069]FIG. 2a is a graph of Peak Transition Energy v. Quantum WellThickness, at 300K, for an In concentration of 0.67 of an InGaAsstrained quantum well on a GaAs substrate and also applying the effectsof (1) the inclusion of superlattice structures, (2) utilizing ˜0.5%nitrogen (N) in the strained quantum well, (3) utilizing a 25 meV gainoffset and ˜50K temperature rise, and (4) utilizing strain compensationand/or alternative substrate orientation;

[0070]FIG. 2b is a schematic illustration of the effect on peaktransition energy, at 300K, by (1) the inclusion of superlatticestructures, (2) utilizing ˜0.5% nitrogen (N) in the strained quantumwell, and (3) utilizing a 25 meV gain offset and ˜50K temperature rise;

[0071]FIG. 3 is a table which tabulates the properties of Inconcentration, strain, calculated critical thickness (CT), peaktransition energy at CT, peak transition wavelength at CT, and otherproperties of the strained InGaAs structures illustrated in FIG. 1;

[0072]FIGS. 4a through 4 d illustrate four examples of superlatticestructures which may be utilized in conjunction with the InGaAs materialof FIG. 1;

[0073]FIGS. 5a through 5 g illustrate different design strategies forutilizing compressive and tensile strain to grow pseudomorphicstructures beyond the CT;

[0074]FIG. 6 is a table which tabulates the strain, CT and the criticalaccumulated strain for GaAsP on GaAs where the concentration of Pvaries;

[0075]FIG. 7 is a graph of peak transition energy v. quantum wellthickness for strained InAs quantum wells on a InP substrate, at 300K,and also applying the effects of 1) utilizing ˜0.54% nitrogen, 2)utilizing ˜20% Sb, and 3) utilizing a 25 meV gain offset and ˜50Ktemperature rise;

[0076]FIG. 8 is a cross section of an active region which incorporatesthe teachings of (1) the inclusion of superlattice structures (2)utilizing nitrogen (N) in the strained quantum well, and (3) utilizingstrain compensation in pseudomorphic InGaAs/GaAs heterostructures;

[0077]FIG. 9a is cross section of a VCSEL which incorporates the quantumwells of FIGS. 1, 2a, 7 and/or 8;

[0078]FIG. 9b is an exploded view of the active region of FIG. 9a;

[0079]FIG. 10a is a cross section of an in-plane emitter whichincorporates the quantum wells of FIGS. 1, 2a, 7 and/or 8;

[0080]FIG. 10b is an exploded view of the active region of FIG. 10a; and

[0081]FIG. 11 is a graph of peak transition energy and peak transitionwavelength v. lattice constant for a variety of binary and ternary groupIII-V semiconductor compounds.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0082] It is advantageous to define several terms before describing theinvention. It should be appreciated that the following definitions areused throughout this application.

[0083] The term “GaAs” refers to a semiconductor composition which maybe used as a substrate. Nominally, the prototypical III-V binarysemiconductor material consisting of equal parts of the two elements Gaand As are used to form the semiconductor material. It should beappreciated that some deviations, to meet device needs or unwantedimpurities, may be permitted which continue to use established GaAsfabrication procedures. To permit for anticipated need for impurities orother relatively insignificant modifications, it is prescribed that bothGa and As are present and combine to form an amount of at least 95% ofthe substrate's entire composition. GaAs has a lattice constant of about5.65 Å. An In_(0.3)Ga_(0.5)As semiconductor has a lattice differing fromthat of GaAs by 0.36% In this application, any substrate having alattice constant which is within 0.36% of that of GaAs, i.e., between5.63 Å and 5.67 Å, is viewed as being within the scope of the invention.Additionally, it should be appreciated that the term “substrate” mayinclude any material underneath the active layer. For example, mirrorlayers, waveguide layers cladding layers or any other layer which ismore than twice as thick as the active layer.

[0084] The term “InGaAs” refers to a semiconductor material comprisingat least In, Ga, and As.

[0085] The term “superlattice” refers to a structure having an averagecomposition but with non-uniform layering, and is usually described interms of atomic monolayer compositions. For example, In

0.0

Ga_(0.25)As could be a homogeneous alloy having the indicatedcompositions, or it could be in the form of a superlattice. Onesuperlattice form may be a periodic structure with each periodcomprising three monolayers of InAs and one monolayer of GaAs.Alternatively, each period may comprise two monolayers of InAs and twomonolayers of In_(0.5)Ga_(0.5)As. The discerning feature of asuperlattice is that the composition be interhomogeneous with respect toatomic layers on a fine scale. Herein, “superlattice” is understood tobe a structure grown on a thick substrate and comprising at least twoadjacent atomic monolayers which differ in at least one constituentelement by at lest 15%. We further point out that the literature usesthe term “superlattice” in several contexts, which may lead toconfusion. For example, in FIG. 4 in the chapter by Coleman,“superlattice” is used in the context of a structure with alternatinglayers but not having a thick substrate to set the required latticeconstant of grown material. Thus, curve (a) in his FIG. 4 is labeled“superlattice” and has a critical thickness twice that of a “quantumwell.” In the context of this application, such a structure would beconsidered to have been grown without a thick substrate and is not partof the present invention. On the other hand, Gourley et al., reports anincrease in critical thickness by more than 25% of superlatticescompared to InGaAs alloys. The superlattices of Gourley el al., areconsistent with the use of the term “superlattice” herein. Thus, it iscontemplated that a superlattice may allow a 25% increase in criticalthickness over the critical thickness for a homogeneous alloy.

[0086] The term “Critical Thickness” or CT, discussed in greater detailbelow, is generally referred to as the critical thickness based on thecriteria developed by Matthews and Blakeslee. For a detailed discussionof Critical Thickness, the reader is referred to the article entitled“Defects in Epitaxial Multilayers: I Misfit Dislocations,” published inthe Journal of Crystal Growth, vol. 27, pp. 118-125 (1974) or thepreviously referenced book by Coleman. For materials epitaxially grownon (100) oriented GaAs and in the absence of strain compensation, thevalue of critical thickness may be determined by the following formula:

CT=(0.4374/f)[In(CT/4)+1]  (1)

[0087] where f is the lattice mismatch normalized to the GaAs latticeconstant, or more commonly referred to as strain. The strain may bedetermined by the portion of In or other elements present in the InGaAssemiconductor material. For example, for In_(y)Ga_(1-y)As on GaAs, fequals 0.07164 multiplied by y, where y varies between 0.0 and 1.0.Generally, the CT is measured in Å and varies between 71.9 Å and 13.6 Åfor In_(y)Ga_(1-y)As where y varies between 0.33 and 1.0.

[0088] For a strained In_(y)Ga_(1-y)As semiconductor material on a InPsubstrate, the equivalent expression is:

CT=(0.454/f)[In(CT/4.15)+1]  (2)

[0089] where f is the lattice mismatch normalized to the InP latticeconstant, or more commonly referred to as strain. This expression is formaterials epitaxially grown on (100) oriented InP and in the absence ofstrain compensation. The strain may be determined by the portion of Inor other elements present in the InGaAs semiconductor material. Forexample, for In_(y)Ga_(1-y)As on InP, f equals 0.032368×[(y−0.53)/0.47],where y varies between 0.0 and 1.0. When y=0.53, the In_(y)Ga_(1-y)As islattice matched to InP. Henceforth, the term CT on nominal GaAs or In Psubstrates shall refer to the solution of equation (1) or (2),respectively. The term CT may also apply to the solution of equationsequivalent to (1) and (2) for materials grown on other substrates. Itshould be appreciated that the term “Critical Thickness” is moregeneral, but less precisely defined.

[0090] Additionally, it should be appreciated that the CT's as definedin equations (1) and (2) are valid for strained-layer structures on athick substrate and having a sufficiently thick overlayer(s) ofunstrained material grown on top. This is the structure used for mostdevice applications. As described by Matthews & Blakeslee, and also byColeman and by Vawter et al., absence of the thick substrate results ina doubling of the critical thickness, while absence of the overlayerhalves the critical thickness. Vawter et al., in an article entitled“Useful Design Relationships for the Engineering of ThermodynamicallyStable Strained-layer Structures,” Journal of Applied Physics, vol. 65,pp. 4769-4773, 1989, discusses the minimum required thickness of theoverlayer(s). This reference is hereby incorporated by reference.

[0091] In this context, the term “pseudomorphic” is used to describe asemiconductor material which is substantially free of misfit orthreading dislocations and being constrained to the lattice constant ofthe substrate in the transverse direction, i.e., horizontal direction.Generally, for the purposes of this application, well-grownsemiconductor layers which have a thickness below their respective CTswill be pseudomorphic. Additionally, by utilizing the teachings, one mayconstruct a pseudomorphic semiconductor material which is above itsrespective critical thickness while maintaining the level of misfitdislocations which would be present if the semiconductor material had athickness below the CT.

[0092] Before continuing with the definitions of certain terms, it isessential to explain how dislocations may be detected. In order to testfor dislocations, a number of techniques are know in the semiconductorart. For example, Gourley et al. in an article entitled “Controversy ofCritical Layer Thickness for InGaAs/GaAs strained-Layer Epitaxy,” Appl.Phys. Lett. 52 (5), pp. 37-379, Feb. 1, 1988, describes the use ofphotoluminescence microscopy (PLM) to detect “dark lines” which resultfrom dislocations. The Gourley et al. article is hereby incorporated byreference. In fact, Gourley used PLM to determine whether or not astrained layer was grown above its critical thickness. PLM may be useddirectly and nondestructively on VCSELs. Since EELs usually have ametallic contact over the active region, the contact must be removed inorder to test the active region. Alternatively, for VCSELs or EELs, thedevice may be removed from its package, allowing optical access from thebottom of the device. Since both GaAs and InP substrates are fairlytransparent to wavelengths over 1 μm, there is no need to remove thesubstrate. The presence of a single dark line is a VCSEL or EEL deviceis sufficient to determine that the device is not pseudomorphic.

[0093] Other techniques are available and are known in the semiconductorart. Electron beam induced current (EBIC) is another test which isnondestructive for VCSELs. EBIC detects dislocations in the activeregion. As with PLM, a single dislocation detected by EBIC is sufficientto determine that the structure is not pseudomorphic. High resolutionelectron microscopy (HREM) allows sufficiently high magnification toobserve atomic dislocations as described by Fang & Morkoc, which isentitled “Integrated Optoelectronics,” Academic Press, pp. 170-173,1995. It should be appreciated that under long-term or high-stressoperation, “dark lines” or dislocations will develop in anysemiconductor lasers or LEDs. For the purposes of this discussion, it isassumed that the testing for dislocations is performed under conditionsin which the device has undergone normal operation for 1,000 hours orless.

[0094] Turning back to the definitions, the term “peak transitionenergy,” usually measured in units of eV, refers to the photon energy atwhich luminescence is highest. Energy varies inversely with wavelengthand may be determined by the following formula:

Energy=1.24/Wavelength  (3)

[0095] with energy in electron volts (eV) and wavelength in micrometers.

[0096] The term “peak transition wavelength (energy),” usually ismeasured in μm or nm (eV), and refers to the emission wavelength(energy) of a semiconductor material at which luminescence is highest.It should be appreciated that while a semiconductor material may emit atone peak transition wavelength (energy), there are limited bands oneither side of this peak transition wavelength (energy) in which lightis also emitted.

[0097] It should be appreciated that we may use the term “transitionwavelength (energy)” in the application. Unless specifically pointedout, we intends the term “transition wavelength (energy)” to have themeaning of the term “peak transition wavelength (energy),” definedabove. It should be appreciated that there may be a significantdifference between “peak transition wavelength (energy)” and “emissionwavelength (energy).” Emission wavelength (energy) refers to thewavelength (photon energy) of maximum emission from the overall device.Therefore, these terms are not used interchangeably, unless specificallyenumerated.

[0098] For most forms of in-plane lasers, the emission wavelength(energy) is very close to the peak wavelength (energy) of the opticalgain, which in turn is very close to the peak transition wavelength(energy). For VCSELs and resonant cavity LEDs the emission wavelength(photon energy) may differ significantly from the peak transitionwavelength (energy).

[0099] “Accumulated Strain” (AS) is defined as: $\begin{matrix}{{AS} = {\int{f \cdot {t}}}} & (4)\end{matrix}$

[0100] i.e., the integral over strained material thickness t of thenormalized lattice mismatch f as previously defined. For strained layersof uniform composition, the AS is simply the layer thickness multipliedby f, i.e., ft. The AS is usually is measured in Å%.

[0101] The “Normalized Accumulated Strains” or NAS are the accumulatedstrains divided by the CAS of the compressively strained layer(s). TheNAS does not have any dimensions or units.

[0102] The “Reduced Effective Strain” (RES) is based directly upon itsdefinition in equations (4) and (5) of Vawter et al., “Useful DesignRelationships for the Engineering of Thermodynamically Stable StrainedStructures,” J. Appl. Phys. 65 (12), pp. 4769-4773, Jun. 15, 1989. Hereit is further generalized to be valid for continuously-graded layers. Inthis context, the RES is simply the AS divided by the total thickness ofthe lowest strained layer and all material grown over it. For structureshaving both compressive and tensile stain, the AS may be zero at one ormore levels below the final growth surface. In such cases, the RES isthe AS divided by the total thickness of material above the highest suchzero level of the AS.

[0103] All concentrations for chemical elements are provided in ratioswhich range from 0.0 to 1.0, where 1.0 corresponds to 100% of thatelement. It should also be appreciated that when we discuss an elementin a group III or V semiconductor material, the ratio applies to theconcentration of the elements in either the group III or group Vmaterials and not the entire semiconductor material. For example, an Inconcentration of 0.5 would correspond to 50% In concentration of thegroup III material used to construct the semiconductor material and notto 50% of the entire semiconductor material. This scheme is usedthroughout the application unless specifically enumerated. It should beappreciated that other group elements such as I, II, IV, VI, VII, VIII,transition, or rare-earth elements, in small quantities, may also beutilized in conjunction with the group III/V materials.

[0104] Turning now to the inventive concepts in this application, theconcept of critical thickness will be described in relation to theinvention. The critical thickness of a lattice mismatched, i.e.,strained material, is qualitatively defined as the maximum thicknessthat a strained layer may be realistically grown without incurring alarge number of misfit dislocations. Under optimal and conventionalgrowth conditions, a strained layer which is less than the criticalthickness will remain elastically strained and have a low density ofatomic misfit dislocations, i.e., it will be pseudomorphic. A strainedlayer grown beyond the critical thickness will have a high density ofdislocations and will have a strain which is at least partially relaxed.A quantitative expression for the critical thickness is provided in apaper by Matthews and Blakeslee and which is entitled “Defects inEpitaxial Multilayers: I Misfit Dislocations,” published in the Journalof Crystal Growth, vol. 27, pp. 118-125 (1974). As discussed above, amore lucid discussion of critical thickness is provided in Coleman'sbook. As described by Anan et al. in an article entitled “Critical LayerThickness on (111) B-Oriented InGaAs Heteroepitaxy,” Applied PhysicsLetters, vol. 60, pp. 3159-3161, 1992, the critical thickness for agiven substrate and layer composition may be different for differentsubstrate orientations.

[0105]FIG. 1 illustrates a plot of the room-temperature (300K) peaktransition energy v. quantum well thickness for several Inconcentrations of In_(y)Ga_(1-y)As strained quantum wells on a GaAssubstrate with GaAs barriers. FIG. 1 is read in conjunction with thetable in FIG. 3. As may be seen, there are eight curves, eachcorresponding to a respective reference numeral 10, 12, 14, 16, 18, 20,22 and 24. Curve 10 represents a 33% concentration of In in the type IIIsemiconductor material comprising the In_(y)Ga_(1-y)As strained quantumwell. This corresponds to y having a value of 0.33. As may be seen, thepeak transition energy decreases as the quantum well thicknessincreases. For a In concentration of 0.33, the critical thickness (CT)is 71.9 Å. At the CT, the critical accumulated strain (CAS) is 170 Å%,the peak transition energy is 1.118 eV and the peak transitionwavelength is 1.109 μm. The CAS in Å% is the CT in A multiplied by thestrain in %. It should be appreciate that the CT discussed with regardto FIGS. 1 and 2a are determined by equation 1.

[0106] For ease of reading, curves 26, 28, 30 and 32 are providedbetween curves 10, 12, 14, 16, 18, 20, 22 and 24. Curves 26, 28, 30, and32 correspond to quantum well thicknesses of 0.75 CT, 1.0 CT, 1.5 CT,and 2.0 CT, respectively. FIG. 3 tabulates the respective values for CTequal to 1.0, 1.5 and 2.0 for the curves 10 through 24. Curves 26through 32 allow for the easy determination of peak transition energy atparticular ratios of CT. Thus, the peak transition wavelength may bedetermined at these particular ratios of CT. For example, for y equal to0.33 and a CT ratio of 0.75, the peak transition energy is ˜1.15 eVwhich corresponds to a peak transition wavelength of 1.078 μm. Curves26, 28, 30, and 32 provide estimation of the peak transition energies atthe CT for In concentrations intermediate to those provided in FIG. 3.

[0107] Turning now to curve 12, a 40% concentration of In in the typeIII semiconductor material comprising the In_(y)Ga_(1-y)As strainedquantum well corresponds to y having a value of 0.40. The CT is 55.3 Å.At the CT, the CAS is 158 Å%. For such a quantum well at the CT, thepeak transition energy is 1.080 eV and the peak transition wavelength is1.148 μm.

[0108] Our calculations are in close proximity to Coleman's for Inconcentrations less than 0.5, but has found slightly shorter wavelengthsfor a given In concentration at the CT than Coleman has. As may be seen,longer wavelengths are achievable, but only to about 1.21-1.22 μm for Inconcentrations in the range from 0.6 to 0.8. As discussed by Coleman,the peak transition energy calculations partially depend on conductionand valance band offsets and there is no solid consensus on theseoffsets.

[0109] Turning now to curve 14, a 50% concentration of In in the typeIII semiconductor material comprising the In_(y)Ga_(1-y)As strainedquantum well corresponds to y having a value of 0.50. The CT is 40.4 Å.At the CT, the CAS is 145 Å%. For such a quantum well at the CT, thepeak transition energy is 1.042 eV and the peak transition wavelength is1.190 μm. Curve 16, a 60% concentration of In in the type IIIsemiconductor material comprising the In_(y)Ga_(1-y)As strained quantumwell corresponds to y having a value of 0.60. The CT is 31.0 Å. At theCT, the CAS is 133 Å%. For such a quantum well at the CT, the peaktransition energy is 1.026 eV and the peak transition wavelength is1.210 μm. Turning now to curve 18, a 67% concentration of In in the typeIII semiconductor material comprising the In_(y)Ga_(1-y)As strainedquantum well corresponds to y having a value of 0.67. The CT is 26.2 Å.At the CT, the CAS is 126 Å%. For such a quantum well at the CT, thepeak transition energy is 1.019 eV and the peak transition wavelength is1.217 μm. The remaining curves follow a similar trend until the Inconcentration is 100% for the group III material as illustrated by curve24.

[0110] Prior reports such as Coleman's, disclose edge-emitting lasersgrown and fabricated with strained InGaAs quantum wells. The quantumwells had 25% In, for which the CT is ˜105 Å. Lasers with 100 Å quantumwells showed excellent threshold characteristics and reliability. It hasbeen found that by increasing the thickness of the quantum wells to 125Å, initial current thresholds were reasonable but the current thresholdsdoubled after testing for times on the order of a few thousand hours.The increase in thickness resulted in a high density of atomic misfitdislocations and associated problems.

[0111] We have determined that merely increasing the In concentrationalone will not allow one to develop a pseudomorphic material on GaAswhich has a peak transition wavelength of 1.3 μm or greater. This isbecause one is not able to reduce the peak transition energy to 0.954 eVor below by merely adjusting the concentration of In. The reader isreferred to column 7, E(CT), of FIG. 3 and also to curve 28 of FIG. 1which provides the peak transition energy, at CT, for differentconcentrations of In. We have found that the minimum peak transitionenergies are obtained for In concentrations in the 0.6 to 0.8 range.This contradicts the notion that by merely increasing the Inconcentration, one would achieve a lower peak transition energy. Forexample, a 100% In concentration quantum well at its CT yields a higherpeak transition energy than would a 50% In concentration quantum well,as may be seen in FIG. 3.

[0112] For ease of reading, FIG. 1 is provided with a scale for selectedpeak transition wavelengths on the right margin of the Figure. Forconvenience, there is a dashed line 11 which represents a 1.3 μmemission wavelength. As discussed above, it is not possible to achieve1.3 μm emission by merely increasing the In concentration while havingthe active region below its respective CT. This is illustrated in FIG. 1by the intersection of line 28 and curves 10,12, 14, 16, 18, 20, 22 and24. It should be appreciated that these respective intersection pointsare always above dashed line 11. Thus, merely increasing Inconcentration will not provide the desired emission wavelength.

[0113] After extensive work, it has determined that it is possible toreduce the emission energies of pseudomorphic quantum wells on GaAssubstrates to 0.954 eV or below, by any of the following procedures orcombinations thereof: (1) the inclusion of superlattice structures withappropriately high In concentrations as the active layer, (2) utilizingstrain compensation to increase the usable layer thickness above the CTfor a InGaAs pseudomorphic active layer in the active region, and (3)utilizing small concentrations, <1%, of nitrogen (N) in thepseudomorphic InGaAsN/GaAs active layer results in a device which iscapable of peak transition wavelengths of 1.3 μm or greater. In all ofthe above techniques, gain offset, via appropriate DBR spacing, may beutilized in VCSELs to detune the optical cavity to resonant energieslower than the peak transition energy by typically 25 meV or even by 50meV or more. Gain offset may also be utilized in some forms of in-planelasers. Furthermore, elevated temperatures may be used to decrease peaktransition energy by about 0.5 meV/K. Additionally, “tilted” or“misoriented” substrates may be used to extend critical thickness asdiscussed below.

[0114] Next, the procedures identified above will be discussed inconjunction with the invention. After each discussion, an exemplarydescription of devices constructed in accordance with the teachings ofthe invention will be provided.

Superlattice

[0115] A superlattice is a plurality of superimposed semiconductorlayers, each as thick as several nm or as thin as one atomic monolayer.In general, by replacing a uniform ternary alloy by a superlattice ofsuccessive periods of binary monolayers, one may produce a superlatticehaving the average composition of the uniform alloy, but with reducedpeak transition energy. When an alloy of InGaAs is replaced with asuperlattice comprising InAs and GaAs, the reduction of peak transitionenergy is sufficient such that combination with gain offset or withanother measure permits at least a 1.3 μm emission wavelength. Previousefforts to use a superlattice for this purpose have been unsuccessfuland further attempts apparently abandoned.

[0116] Turning now to FIG. 2a, a graph of peak transition energy v.quantum well thickness is shown. Curve 18, is illustrated as a nominalcurve which will be affected by the inventive techniques. As may be seenin FIG. 2b, the transition energy may be reduced by includingsuperlattice structures. With appropriately high In concentrations inthe active layer, emission wavelengths greater than or equal to 1.3 μmmay be achieved. In combination with this technique, gain offset ortemperature rise may be utilized. As may be seen, a reduction of about40 meV may be achieved by utilization of a superlattice structure asdescribed below. The effect of utilizing only a superlattice isillustrated in FIG. 2a as curve 40. An additional decrease in transitionenergy, typically 25 meV or even more, is possible by utilizing gainoffset or a temperature rise as illustrated in FIG. 2b. The peaktransition energy decreases by about 0.5 meV/K. Thus, a 50K rise intemperature produces about a 25 meV drop in the peak transition energy.It should be appreciate that the effects of gain offset or temperaturerise may be used in conjunction with each other or with any othertechnique described in this application. Curve 42, illustrates thecumulative effect of utilizing a superlattice with gain offset ortemperature rise. As may be seen, by the intersection of curve 42 andline 28′, the desired emission wavelength of 1.3 μm, as illustrated byline 11, is achievable while maintaining the active region below itsrespective CT. Lines 26′, 30′ and 32′, similarly illustrate values ofthe 0.75 CT, 1.5 CT, and 2.0 CT, respectively. Now, we will discuss thespecific superlattice structures which may be utilized in conjunctionwith the invention.

[0117] The concentrations of In selected for curves 10 through 24 ofFIG. 1 were selected to match the concentrations achievable in binarysuperlattices of InAs-GaAs with integral multiples of monolayers foreach. For example, the 0.67 concentration of In could be an alloy ofIn_(0.67)Ga_(0.33)As or it could be a superlattice whose periodcomprises 2 monolayers of InAs followed by 1 monolayer of GaAs. It hasbeen shown by Roan et al. that a binary superlattice may have a lowerpeak transition energy and longer peak transition wavelength than analloy having the same In concentration. Roan et al. has reported a 117meV reduction in peak transition energy in a (InAs)₁(GaAs)₁superlattice, but, Roan et al. believes that this value is not dueexclusively to the superlattice structure. Additionally, to ourknowledge, Roan et al. has abandoned further researching this approach.We believe that this value is too high and is at least partly due tostrain relaxation caused by misfit dislocation formation and that thesuperlattice-related shift will be approximately 40 meV instead of 117meV. A 40 meV shift attributed to superlattice ordering is reported byKaliteevski et al., in an article entitled “Bandgap Anomaly andAppearance of a Monolayer Superlattice in InGaAs Grown by Metal OrganicChemical Vapour Deposition,” Semicond. Sci. Technol. 10, pp. 624-626(1995). With a 40 meV shift, a binary super-lattice of (InAs)₂(GaAs)₁ atthe CT will have a peak transition energy of 0.979 eV as may be seen bythe intersection of curve 40 with line 28′ in FIG. 2a. This superlatticestructure is illustrated in FIG. 4a. As may be seen, this structure isillustrated for an In concentration of 0.67. For clarity, only one atomof In has been labeled as reference numeral 34 in the superlattice. In asimilar fashion, Ga has been labeled as reference numeral 36 while As isreference numeral 38.

[0118] One period of the (InAs)₂(GaAs)₁ superlattice, where y=0.67, isabout 8.5 Å and the calculated CT is 26.2 Å. Thus, three periods arejust below the CT. It should be appreciated that two periods yield amuch thinner quantum well with a much shorter peak transitionwavelength. For a (InAs)₃(GaAs)₁ superlattice, where y=0.75, one periodis about 11.3 Å and the calculated CT is 22 Å. Thus, two periodsslightly exceed the calculated CT by 0.6 Å. This superlattice structureis illustrated in FIG. 4b. For such thin quantum wells, even thethickness of the well is uncertain since exclusion of one GaAs monolayeroutside of the well would reduce the well thickness by about 3 Å. Inboth the 0.67 and 0.75 examples, a 3 Å reduction in thickness brings thequantum well below the CT. It should be appreciated that the criticalthickness for a superlattice may actually exceed the CT. Gourly et al.found that the critical thickness for a superlattice was measurablylarger, by more than 20%, than the critical thickness for a ternaryalloy having the same average composition.

[0119] For a superlattice structure of (InAs)₄(GaAs)₁, corresponding toy=0.8, two periods greatly exceed the CT. A single period of thesuperlattice is simply a InAs single quantum well and the superlatticediscussion is not appropriate. Thus, there are at least two periodicbinary superlattices which have a peak transition wavelength in theneighborhood of 1.3 μm while keeping below the CT. The first is a(InAs)₂(GaAs)₁ with three periods and the second is a (InAs)₃(GaAs)₁with two periods. It should be appreciated that even these twosuperlattices alone do not achieve the desired wavelength of 1.3 μm butmerely get close to this result. A means for achieving 1.3 μm emissionwavelength will be discussed in greater detail below.

[0120] It should be appreciated that for a superlattice structure withan In concentration of 0.9 or 1.0, it is inappropriate to utilize abinary superlattice structure absent the use of strain compensation.This is because for example, if the In concentration was 0.9, therewould be nine monolayers of In and this structure would be well abovethe CT. For the case where the In concentration is 1.0, there would beno Ga.

[0121] Turning to FIGS. 1, 2a, 2 b and 3, the inventive concept ofutilizing the superlattices, described above, in conjunction with gainoffset to produce an emission wavelength of 1.3 μm will be discussed.

[0122] As discussed above, it has been determined that the minimum peaktransition energies are obtained for In concentrations between andincluding 0.6 and 0.8. In an attempt to be comprehensive, weadditionally discuss the In concentration of 0.5 for curve 14. It shouldbe appreciated that these discussions may also apply to curves 10, 12and 24.

[0123] Using the E(CT) column of FIG. 3 and curve 14 of FIG. 1, the peaktransition energy with an In concentration of 0.5 is 1.042 ev for analloy of In_(0.5)Ga_(0.5)As. With the use of a superlattice as describedabove, this peak transition energy may be reduced to 1.002 eV. Thus, again offset of 48 meV below the peak transition energy is required foran emission wavelength of 1.3 μm, corresponding to a the peak transitionenergy of 0.954 eV. This gain offset is very large and may be too largeto be practical in a functioning device unless it operates at elevatedtemperatures as discussed below.

[0124] Using the E(CT) column of FIG. 3 and curve 16 of FIG. 1, the peaktransition energy with an In concentration of 0.6 is 1.026 ev for analloy of In_(0.6)Ga_(0.4)As. With the use of a superlattice as describedabove, this peal transition energy may be reduced to 0.986 eV. Thus, again offset of 32 meV below the peak transition energy is required foran emission %wavelength of 1.3 μm, corresponding to a the peaktransition energy of 0.954 eV. This gain offset is reasonable in afunctioning device.

[0125] Using the E(CT) column of FIG. 3 and curve 18 of FIG. 1, the peaktransition energy with an In concentration of 0.67 is 1.019 ev for analloy of In_(0.67)Ga_(0.33)As. With the use of a superlattice asdescribed above, this peak transition energy may be reduced to 0.979 eV.Thus, a gain offset of 25 meV below the peak transition energy isrequired for an emission wavelength of 1.3 μm, corresponding to a thepeak transition energy of 0.954 eV. This gain offset is very reasonablein a functioning device.

[0126] Using the E(CT) column of FIG. 3 and curve 20 of FIG. 1, the peaktransition energy with an In concentration of 0.75 is 1.018 ev for analloy of In_(0.67)Ga_(0.33)As. With the use of a superlattice asdescribed above, this peak transition energy may be reduced to 0.978 eV.Thus, a gain offset of 24 meV below the peak transition energy isrequired for an emission wavelength of 1.3 μm, corresponding to a thepeak transition energy of 0.954 eV. This gain offset is very reasonablein a functioning device.

[0127] Using the E(CT) column of FIG. 3 and curve 22 of FIG. 1, the peaktransition energy with an In concentration of 0.80 is 1.021 ev for analloy of In_(0.8)Ga_(0.2)As. With the use of a superlattice as describedabove, this peak transition energy may be reduced to 0.981 eV. Thus, again offset of 27 meV below the peak transition energy is required foran emission wavelength of 1.3 μm, corresponding to a the peak transitionenergy of 0.954 eV. This gain offset is very reasonable in a functioningdevice.

[0128] It is interesting to note that after an In concentration of 0.75,the peak transition energy begins to increase with increased Inconcentration. In the case where y=0.8, the increase in the peaktransition energy may still be compensated for by utilizing anappropriate superlattice and gain offset as discussed above. After y isgreater than 0.8 it becomes increasingly more difficult to compensatefor the increase in peak transition energy by selecting an appropriatesuperlattice and gain offset.

[0129] There are also a limited number of aperiodic superlattices whichmay accomplish a similar result of a peak transition wavelength in theneighborhood of 1.3 μm while keeping below the CT. For example, astructure comprising:

(InAs)₁(GaAs)₁(InAs)₂(GaAs)₁(InAs)₂(GaAs)₁(InAs)₁

[0130] bounded by GaAs on both sides and denoted by the formulaI₁G₁I₂G₁I₂G₁I₁ accomplishes the required result. This aperiodicsuperlattice is illustrated in FIG. 4c and has an In concentration of67%. For y=0.67, one period of the I₁G₁I₂G₁I₂G₁I₁ superlattice, thecalculated CT is 26.2 Å is about this thickness. The peak transitionenergy for this quantum well superlattice is estimated to be 0.979 eV.

[0131] Another example is the aperiodic superlattice(InAs)₁(GaAs)₁(InAs)₄(GaAs)₁(InAs)₁ bounded by GaAs on both sides anddenoted by the formula I₁G₁I₄G₁I₁ which also accomplishes the requiredresult. This aperiodic superlattice is illustrated in FIG. 4d and has anIn concentration of 60%. For y=0.60, one period of the I₁G₁I₄G₁I₁superlattice, the calculated CT is 31 Å and is about this thickness. Thepeak transition energy is calculated to be 0.986 eV.

[0132] Yet another example is the aperiodic superlattice(InAs)₂(GaAs)₂(InAs)₄ (GaAs)₂(InAs)₂ bounded by GaAs on both sides anddenoted by the formula I₂G₃I₂ which also accomplishes the requiredresult. This aperiodic superlattice has an In concentration of 60%. Twoperiods of the I₂G₂I₄G₂I₂ superlattice is about 30 Å and the calculatedCT is 31 Å. Thus, two periods are well below the CT. The peak transitionenergy for this quantum well superlattice is calculated to be 0.996 eV.Yet another example is I₂G₁I₃G₁I₂ having an average In concentration of70%.

[0133] In the preceding examples, the determinations of the average Inconcentrations include the presence of ½ monolayer of GaAs on eitherside. It should be appreciated the above superlattices are merelyexemplary and that there are other superlattices, including non-binarysuperlattices, which may be used in conjunction with the invention. Forexample, a superlattice comprising alternating monolayers of InGaAshaving 50% In and InAs has an average In concentration of 75%. Also,superlattices comprising other elements, such as, but not limited to Sb,may be used. The requirements of any superlattice which is contemplatedby this invention is a superlattice with the following characteristics:(1) the overall lattice must have a sum of In and Sb concentrations of0.5 or greater; (2) the strained layers used for the formation of thesuperlattice must have a thickness below the CT for the associatedsemiconductor material; and (3) the peak transition energy must besufficiently low, e.g., 1.0 eV, such that the inclusion of gain offsetin VCSELs and/or elevated temperature results in an emission wavelengthof 1.3 μm or greater for devices on GaAs substrates and 2.5 μm orgreater for devices on InP substrates.

Strain Compensation

[0134] Developments in epitaxial growth techniques such as molecularbeam epitaxy (MBE) and organometallic vapor phase epitaxy (OMVPE) havemade it possible to grow multi-component epitaxial heterostructures withprecise composition and thickness control. These new methods have pavedthe way for growing pseudomorphic lattice-mismatched strained structuresdifferent from those possible at equilibrium. Consequently, artificiallylayered structures that are lattice mismatched may be realized andinvestigated but the degree of usable mismatch and thickness are subjectto limitations such as those described by Matthews & Blakeslee.

[0135] The mechanisms describing epitaxial growth on a substrate with adifferent lattice constant are very much dependent on the materialsinvolved and details of the overall structure. The lattice-mismatchedlayer is assumed to be strained coherently without the generation oflarge amounts of misfit dislocation defects under appropriate growthconditions and if the layer thickness is kept below a criticalthickness. The strain in a layer with a lattice deformation of Δa is:

f=Δa/a ₀  (5)

[0136] where a₀ is an undeformed lattice constant. The misfit strainenergy increases with the thickness of a strained layer. When thethickness of a strained layer reaches a critical value, it becomesenergetically favorable for misfit dislocations to be generated. Thestrain will be gradually relaxed by misfit dislocations as the layerthickness increases. Dislocations in an active layer degrade deviceproperties as discussed above. Therefore, the prior art has appreciatedthat it is important to grow strained layers within the CT. Otherwise,the strained layer would be partially or completely relaxed.

[0137] If a single quantum well is grown at nearly the CT, followed bythe growth of a thin unstrained barrier, and then by a second, similarquantum well, misfit dislocations will occur as discussed above. This isbecause the strain forces of the two quantum wells accumulate to exceedthe allowed strain force. In cases where it is desirable to havemultiple quantum wells, each with maximum strain, a technique called“strain compensation” is used to grow large numbers of such wells insuccession. In strain compensation, the accumulation of compressivestrain forces are balanced by the introduction of tensile strain betweenthe wells. For this discussion, it is assumed that the quantum wells arecompressively strained and the surrounding barriers are tenselystrained. However, it should be appreciated that strain compensation maybe used in the opposite manner also, i.e., the quantum wells are tenselystrained and the surrounding barriers are compressively strained. Use ofstrain compensation to increase the number of strained quantum wellswhich may be grown without dislocations is frequently used in the priorart and is described by Zhang and Ovtchinnikov in an article entitled“Strain-compensated InGaAs/GaAsP/GaInAsP/GaInP Quantum Well Lasers(λ˜0.98 μm) Grown by Gas-Source Molecular Beam Epitaxy,” Applied PhysicsLetters, vol. 62, pp. 1644-1646, 1993, which is hereby incorporated byreference, see also U.S. Pat. No. 5,381,434 by Bhat and Zah.

[0138] It is also possible to use strain compensation to grow a strainedquantum well up to twice its critical thickness without creating misfitdislocations as described by Hoke in U.S. Pat. No. 5,060,030, which ishereby incorporated by reference. To grow an increased thicknessstrained quantum well, a layer of oppositely strained material is grownfirst. For example, to grow an increased-thickness compressivelystrained quantum well, a layer of tensely-strained material is grownfirst. Then, when the compressively-strained layer is grown to itsnominal CT, the accumulated strain of the tensely-strained layer issubtracted from the accumulated strain in the well, reducing the forceswhich generate misfit dislocations. To a first approximation, acompressively-strained layer may be grown to about twice its nominalcritical thickness before the accumulated strain force is once again atthe level which will form dislocations. Not disclosed by Hoke, butpreferably included, is a second oppositely-strained layer is grown overthe quantum well to bring the accumulated strain once again toapproximately zero.

[0139] Thus, small-lattice-constant material layers, consequently intension as epitaxially grown on GaAs based material may average outcompressive strain in the large lattice-constant emitter layer. Such“tensely strained” layers may be functionally inert spacers or may bepart of mirror or other functioning parts of the structure. They mayserve to permit increased thickness of a homogenous emitter layer or maybe used to increase the number of quantum wells in a Multiple QuantumWell (MQW) structure.

[0140]FIG. 5 schematically illustrates the use of strain compensation togrow pseudomorphic semiconductor layers above their nominal criticalthickness, e.g., above the CT. In FIG. 5, the vertical axis representsthe lattice constant of the growth material, with the horizontal axispositioned at the lattice constant of the substrate. Thus,compressively-strained material is represented above the horizontal axisand tensely-strained material below the axis. The horizontal axisrepresents the growth direction. An excessive accumulation ofcompressive or tensile strain above the critical accumulated strain forthe material being grown (CAS column in FIG. 3) represents the onset ofmisfit dislocations. In FIG. 5, the normalized accumulated strains (NAS)are the accumulated strains divided by the CAS of the compressivelystrained layer(s).

[0141] For a strained layer as in FIG. 5a, its maximum thickness is theCT and the NAS is +1. In FIG. 5b, a tensely-strained layer is grown toan arbitrary thickness (but approximately equal to or less than its CT)with a NAS of −X, followed by growth of a compressively-strained layerhaving a NAS of 1+X. The total NAS is +1. Nowhere in the growth does themagnitude of the total NAS exceed 1. An extension of this concept isillustrated in FIG. 5c in which the total NAS is approximately zero.Following the growth of a tensely-strained layer with NAS of −X, acompressively-strained layer with NAS of +2X is grown, bringing the NASto +X. This layer is followed by growth of another tensely-strainedlayer with N NAS of −X, so that the final NAS is zero. In all of theabove examples, when X is >0.5, the compressively-strained layerthickness exceeds its CT and when X=1 the compressivley strained layeris grown to twice its CT.

[0142] It is furthermore possible to have a NAS <−1 for thetensely-strained layer while still keeping the tensely-strained layerbelow its critical thickness. This occurs when the magnitude of thetensile strain is less than that of the compressive strain because alayer having a smaller strain has a larger CAS (see FIGS. 3 and 6). Thismay allow the pseudomorphic growth of layers to more than twice theirCT.

[0143] Rather than having abrupt interfaces between the compressively-and tensely-strained layers as shown in FIGS. 5b and 5 c, it may beadvantageous to have graded interfaces as shown in FIGS. 5d, 5 e, and 5f. FIG. 5d shows a stepped interface; FIG. 5e shows a superlatticeinterface; and FIG. 5f shows a smoothly graded interface. Steppedinterfaces, and especially the smoothly graded interfaces, minimize thelocal strain forces at any location and therefore minimize thelikelihood of dislocations. Thus, a quantum well may actually be grownto more than its CT without creating high densities of misfitdislocations. Multiple quantum wells, each significantly above theircritical thickness, may be grown using the strain-compensating structurerepresented in FIG. 5g. Although stepped interfaces are illustrated inFIG. 5g, abrupt or any other graded interface may be used in conjunctionwith the inventive concept. In FIG. 5g, the final NAS is zero throughoutthe entire growth and at no level does the AS exceed the CAS for thematerial at that level. It should be appreciated that the final NAS doesnot need to be exactly zero. It should be appreciated that the shape ofthe graded interfaces in FIGS. 5d, 5 e, and 5 f are merely illustrativeof the concept of graded interfaces. The invention contemplates anygraded interface which minimizes the overall strain of the respectivelayers as within the scope of the invention.

[0144] As discussed above, pseudomorphic strained layers exceeding theCT may be grown. Strained as-deposited layers in the pseudomorphic statemay relax if the thermal treatment associated with device growth orfabrication is too high. Strained-layer breakdown is most directlydetermined by an excess stress (the difference between that due tomisfit strain and that due to dislocation line tension) and temperature.Elevated temperatures provide additional lattice energy to help overcomethe interatomic forces which would otherwise keep the lattice in itspseudomorphic state. This strain layer breakdown may be prevented by:(1) providing strain compensation as discussed above and throughout theentire growth process, maintaining the accumulated strain below thecritical accumulated strain of the material being grown; and (2)maintaining the temperature of the growth material and other growthparameters at appropriate levels for the method of growth and thestrain. It should be appreciated that both strain and method of growthwill affect the maximum usable growth temperature.

[0145]FIG. 3 shows that the accumulated strain of a singleIn_(y)Ga_(1-y)As quantum well at its CT, i.e., the CAS, varies with thelattice mismatch. The same is true for any strained layer. The forceexerted by the misfit strain is proportional to the accumulated strain(AS) as defined previously, and this is the mechanism behind the linearterm in the CT equation above, i.e., the left hand side of equations 1and 2. The right had side of equations (1) and (2) denotes the linetransition of dislocation which acts to resist the formation ofdislocations.

[0146]FIG. 6 provides data for GaAs_(1-z)P_(z) for z, the strain, CT,and accumulated strain, respectively in the four columns. Differentdesign strategies are now described by means of examples. The examplesare by no means meant to limit the scope of the invention, only tobetter communicate its implementation. Furthermore, although thestructures are described in terms of having abrupt interfaces of thetype illustrated in FIGS. 5a, 5 b and 5 c, it is understood that otherinterface types may be employed in the same fashion, such as theinterface types illustrated in FIGS. 5d, 5 e, 5 f, and 5 g as well asthe other alternatives described above. The AS, since it is defined asan integral, is valid for any form of interface or quantum well profileincluding but not limited to: steps, grades, and superlattices.

[0147] The accumulated strain AS and strain compensation are most easilyunderstood when the tensile and compressive strained materials have thesame magnitude of lattice mismatch, but opposite signs. The firstexample is In_(y)Ga_(1-y)As with y=0.5 and GaAs_(1-z)P_(z) with z=1.0.The latter is GaP, and both materials have lattice constants differingfrom that of GaAs by about 3.6%. FIG. 3 shows that a quantum well widthof about 80 Å is required for 1.3 μm peak transition with y=0.5In_(y)Ga_(1-y)As, that this is 1.98 times the CT for y=0.5, and that theaccumulated strain is 287 Å%. Thus to grow 1.3 μm emittingIn_(y)Ga_(1-y)As quantum well with y=0.5 while keeping the accumulatedstrain below 145 Å%, the GaP layer is first grown to its CT of 40 Å toproduce an accumulated strain of −144 Å%. Then the y=0.5In_(y)Ga_(1-y)As layer is grown. When it reaches a thickness of 80 Å,the total accumulated strain is (−144+287)Å%=143 Å%, which is just belowthe allowable accumulated strain of 145 Å% corresponding to the CT ofy=0.5 InGaAs. The 2-layer structure would then resemble that of FIG. 5b.Preferably, another symmetric tensile layer of GaP would be grown tobring the total accumulated strain back to approximately zero, similarlyto the illustration of FIG. 5c.

[0148] For 1.3 μm emission, the structure described in the precedingparagraph is undesirable in that the GaP and InGaAs layers are grownnearly to maximum pseudomorphic thicknesses. While such a structure maybe grown, it is preferred to produce structures which do not press soclose to such physical limits. FIGS. 1 and 3 show that 1.3 μm emissionis better reached with higher In concentrations in the In_(y)Ga_(1-y)Aslayer(s).

[0149] The next example, a y=0.6 In_(y)Ga_(1-y)As well is used. Thismaterial has a strain of +4.3%, and a CT of 31 Å at which theaccumulated strain is 133 Å%. For 1.3 μm emission, a well thickness ofabout 44 Å is needed which is 1.42 times the CT. A z=0.7 GaAs_(1-z)P_(z)material is used for this example, which has −2.52% strain, and a CT of66 Å at which the CAS is −166 Å%. The In_(0.6)Ga_(0.4)As quantum wellfor this example is 50 Å thick (1.61 times its CT), having anaccumulated strain of 215 Å%. Its calculated that the peak transitionwavelength firm FIG. 1 is 1.333 μm, though the actual emissionwavelength may be slightly shorter due to the high-bandgapGaAs_(0.3)P_(0.7) barrier layers. To compensate the accumulated strainof the In_(y)Ga_(1-y)As well, GaAs_(0.3)P_(0.7) barriers are designed toproduce −107.5 Å% of accumulated strain on either side of the well; TheGaAs_(0.3)P_(0.7) barriers are thus each 43 Å thick, which is only 0.65times their CT. Nowhere in the growth of this structure does themagnitude of the total accumulated strain exceed 107.5 Å%, i.e., it isalways less than the magnitude of the CAS of either of the strainedmaterials.

[0150] A similar example is now described, which used the same y=0.6In_(y)Ga_(1-y)As well as the previous example. The difference is thatGaAs_(0.5)P_(0.5) material is used for this example, which has −1.8%strain, and a CT of 103 Å at which the accumulated strain is −185 Å%.The GaAs_(0.5)P_(0.5) barriers to produce −107.5 Å% of accumulatedstrain to either side of the well are each 60 Å thick, which is only0.58 times their CT. Nowhere in the growth of this structure does themagnitude of the total accumulated strain exceed 107.5 Å%, i.e., isalways less than the accumulated strain corresponding to the CT ofeither of the strained materials.

[0151] An example is now described using a y=0.8 In_(y)Ga_(1-y)As wellwhich has a strain of +5.73%, and a CT of 19.8 Å at which theaccumulated strain is 114 Å%. A GaAs_(0.7)P_(0.3) material is used forthis example, which has −1.08% strain, and a CT of 199 Å at which theaccumulated strain is −215%. The In_(0.8)Ga_(0.2)As quantum well forthis example is 50 Å thick (2.43 times its CT), having an accumulatedstrain of +287 Å%. Its calculated peak transition wavelength from FIG. 1is 1.59μ, though the actual emission wavelength may be slightly shorterdue to the high-bandgap GaAsP barrier layers. To compensate theaccumulated strain of the In_(y)Ga_(1-y)As well, a 185 Å thickGaAs_(0.7)P_(0.3) barrier is first grown to produce −200 Å% ofaccumulated strain at 0.93 of its CT. The y=0.8 InGaAs well is thengrown. At the end of the In_(y)Ga_(1-y)As growth, the total accumulatedstrain is (−200+287)=+87 Å%, which is well below the criticalaccumulated strain for y-0.6 InGaAs. Preferably, an 80 Å thick z=0.3GaAsP tensile strained layer is grown to bring the total accumulatedstrain to approximately zero. Nowhere in the growth of the y=0.8 welldoes the accumulated strain exceed the well's CAS of +114 Å%, nor duringthe growth of the z=0.3 material does the accumulated strain fall belowits CAS of −215 Å%.

[0152] A final example is now described for a multiple quantum wellstructure using a y=0.6 InGaAs well which has a strain of +4.3% and a CTof 31 Å at which the CAS is 133 Å%. To provide the strain compensation,a z=1.0 GaP material is used, which has −3.6% strain, and a CT of 40 Åat which the CAS is −144 Å%. Each y=0.6 InGaAs quantum well, for thisexample, is about 50 Å thick (1.61 times its CT), having a targetedaccumulated strain of +216 Å%. To compensate the AS of the InGaAs wells,the first z=1.0 GaP barrier is grown 30 Å thick to produce −108 Å% ofaccumulated strain at 0.75 of its CT. The first 50 Å thick y=0.6 InGaAswell is then grown. At the end of this InGaAs growth, the total AS is(−108+216)=+108 Å%. A second GaP barrier is then grown to a thickness of60 Å, contributing −216 Å% to the AS, bringing the AS to −108 Å%. Asecond 50 Å thick y=0.6 InGaAs quantum well is then grown. At the end ofthis InGaAs growth, the total AS is again (−108+216)=+108 Å%. A numberof additional GaP barriers and InGaAs wells may be grown in a similarfashion, for example, producing 7 or more quantum wells, ending with theInGaAs well and an AS of +108 Å%. Preferably, a final GaPstrain-compensating barrier is grown similarly to the initial one, i.e.,30 Å thick to contribute −108 Å% to the AS. In this case, the final ASis close to zero. Nowhere in the growth of the strained materials doesthe AS exceed the CAS of either of the constituent materials.

[0153] For a graphical representation of the effect of straincompensation on an active material, the reader is referred to FIGS. 1,2a and 7. As may be seen, in FIG. 1, by utilizing strain compensation,possibly in combination with appropriate substrate orientation, one isable to grow the active material to 2× the CT or even greater. One maysee by the intersection of line 32 with curves 14, 16, 18, 20, 22 and 24that all of these intersections fall below line 11. Thus, for Inconcentrations equal to or greater than 0.5, it is possible to achievepeak transition emissions of 1.3 μm or greater. Similarly, in FIG. 2a,one may move down respective curves 18, 40, 42, 44, 46, and 48 to line32′ and even to line 32″ if nitrogen is used as discussed below.

[0154] It should be appreciated that strain compensation may be combinedwith the superlattice structures discussed above or any of the othertechniques discussed below. Discussions on strained layer superlatticesare provided by Dagenais et al., which is entitled “IntegratedOptoelectronics,” Academic Press, pp. 175-178, 1995. This book is herebyincorporated by reference. This technique may also be combined withNitrogen-containing active layers as discussed below.

Nitrogen-Containing Active Layers

[0155] Very recently it has been shown that the compound InGaAsN mayhave some favorable qualities for producing 1.3 μm lasers on GaAssubstrates, see Kondow et al., in Japanese Journal of Applied Physics,vol. 35, pp. 1273-1275, 1996. InGaAsN may be thought of as an alloy ofGaAs, InAs and GaN. Compared to the lattice constant of GaAs, GaN has alattice constant about 20% smaller, and InAs has a lattice constantabout 7% larger. Thus, on GaAs substrates the lattice mismatch of GaN isabout 3 times that of InAs and of the opposite sign. Therefore InGaAsNlattice matched to GaAs will have about 3 times as much In as N. Aroom-temperature photoluminescence spectrum of a 70 Å thickIn_(0.3)Ga_(0.7)As_(0.99)N_(0.04) quantum well showed significantbroadening. The peak wavelength of photoluminescence was 1.232 μm, whilethe peak wavelength for a 70 Å thick In_(0.3)Ga_(0.7)As well was 1.088μm. In this report the authors state that their plan to reach 1.3 μmwavelengths is to increase the N content.

[0156] In calculating the CT for nitrogen-containing materials, thedecrease in lattice constant due to the nitrogen should be taken intoaccount. Kondow et al., has shown GaN to have a lattice constant of 4.5Å in the context of being grown on GaAs substrates. The lattice mismatchis thus ˜20.35%, which is about 3 times the lattice mismatch betweenInAs and GaAs and of the opposite sign. Thus, the addition of 1% N tothe InGaAs compound produces a lattice constant approximately equal tothe original InGaAs, with the In content reduced by 3%. For example, theCT of InGaAs with y=0.3 is about 82 Å and the CT for y=0.27 is about 94Å. The CT of an In_(0.3)Ga_(0.7)As_(0.99)N_(0.01)quantum well shouldtherefore be about 94 Å, corresponding to the y=0.27 InGaAs quantumwell.

[0157] Apparently it is difficult to grow laser-quality InGaAs withlarge N contents. Our calculations, described above, favor high Inconcentrations to approach 1.3 μm emission on GaAs substrates.Therefore, contrary to the approach taken by Kondow et al., we havedetermined that 1.3 μm emission would be better achieved by increasingthe In content, rather than the N content. Given the result that a 1% Ncontent decreases the transition energy by 185 meV, as published byKondow et al., it follows that a InGaAs quantum well with a transitionenergy less than 1.139 eV, if modified by replacing 1% of the As with N,would be brought to the desired 0.954 eV transition energy whichcorresponds to a peak transition emission of 1.3 μm. Our calculationsfor a In_(0.3)Ga_(0.7)As_(0.99)N_(0.01) quantum well are as follows. TheCT is about 94 Å, similar to a In_(0.27)Ga_(0.73)As quantum well sincethe nitrogen reduces the lattice constant as discussed above. The peaktransition energy for a 94 Å thick In_(0.27)Ga_(0.73)As quantum well isabout 1.122 eV. Subtracting the 185 meV for the 1% nitrogen yields 0.937eV or a peak emission wavelength of 1.32 μm. Referring again back toFIG. 3, it is seen that a 1.118 eV transition energy may be achievedwith an In_(0.33)Ga_(0.67)As quantum well which is still below the CT.Thus a nominal InGaAs quantum well with 33% or more In may be brought toa peak transition wavelength of ≧1.3 μm by the incorporation of about0.89% N. The In_(0.4)Ga_(0.6)As quantum well with 40% In at its CT has atransition energy of about 1.080 eV, just 126 meV above the desired0.954 eV. Thus, a 0.68% nitrogen concentration would be needed to bringthe quantum well to a 1.3 μm peak transition wavelength. TheIn_(0.5)Ga_(0.5)As well with 50% In at its CT has a transition energy ofabout 1.05 eV, just 96 meV above the desired 0.954 eV. Thus a 0.52% Nconcentration would be needed to bring it to a 1.3 μm peak transitionwavelength. The InGaAs quantum wells discussed earlier with 67%, 75% and80% In had transition energies about 66 meV above the desired 0.954 eV.Thus they may be converted to 1.3 μm emitting quantum wells byincorporating only about 0.35% N. In the above inventive examples, theincrease in CT was not included. Thus, the 1.3 μm peak transitionwavelength may be accomplished with nitrogen levels lower than specifiedin the examples, or at thicknesses below the respective CTs. The use ofnitrogen incorporation in combination with gain offset allows theattainment of 1.3 μm emission wavelengths with peak transition energieshigher than 0.954 eV, by typically 25 meV or even more. This allowsfurther reductions in nitrogen content and/or quantum well thicknesses.

[0158] Turning now to FIGS. 2a and 2 b, the effect of nitrogen isillustrated for a quantum well having 0.67 In concentration. Nominalcurve 18 is modified by the incorporation of 0.5% N to form curve 44.FIG. 2b illustrates that a reduction in transition energy ofapproximately 92 meV is possible by the introduction of ˜0.5% nitrogenat any In concentration. Furthermore, due to the decreased latticeconstant, the CT is increased slightly. The 0.5% nitrogen compositiontherefore increases the CT from 26.2 Å to about 27.2 Å. This isindicated by the vertical dashed line 28″. Lines 26″, 30″ and 32″,similarly illustrate increased values of the 0.75 CT, 1.5 CT, and 2.0CT, respectively. As may be seen from the intersection of curve 44 withline 28″, the transition energy is 0.917 eV and the desired peaktransition wavelength of 1.3 μm is achievable while maintaining theactive region below its respective CT. For VCSELs, an additionaldecrease in emission energy, typically 25 meV, or even more, is possibleby utilizing gain offset as illustrated in FIG. 2b. Curve 46,illustrates the cumulative effect of utilizing 0.5% nitrogen incombination with gain offset.

[0159] Additional reductions of the peak transition energy may beaccomplished by elevating the temperature of the material or device. Thepeak transition energy decreases by about 0.5 meV/K. Thus, a 50K rise intemperature produces about a 25 meV drop in the peak transition energy.Curve 48 in FIG. 2a illustrates the cumulative effect of utilizing 0.5%nitrogen in combination with 25 meV of gain offset and furthermore incombination with 25 meV reduction in peak transition energy due to atemperature rise of about 50K (about 50° C.). It is to be appreciatedthat reduction in peak transition energy of more than 25 meV or lessthan 25 meV may be accomplished via larger or smaller temperature rises,respectively. It is also to be appreciated that the reduction in peaktransition energy via temperature rise may be accomplished independentlyof any of the other techniques or in combination with any of thetechniques discussed in this application. Only a subset of the possiblecombinations are illustrated in FIG. 2a in order to better elucidate theindividual effects and select combinations. Therefore, FIG. 2a is merelyillustrative of these select combinations and is not intended torestrict the inventive concept to merely these combinations. Thus, theuse of small nitrogen contents in combination with any combination orother techniques described herein produce a decrease in the requirednitrogen content, decrease in the required quantum well thickness, orother desirable material attributes.

[0160] Therefore, it has been sown that partial substitution of As by Nincreases peak transition wavelength. Use of small amounts of N, i.e.,less than or equal to 0.95% of the total group V element content,together with substantial In content, greater than or equal to 33% ory=0.33, permits pseudomorphic structures which emit at 1.3 μm orgreater.

Gain Offset

[0161] The term “gain offset” in a VCSEL refers to the non-coincidenceof the cavity resonance from the peak of the photoluminescence orelectroluminescence emission spectrum of the light-emitting activelegion (peak transition wavelength). The full-width-at-half-maximum ofthis spectrum under low excitation is typically about 15 nm at 850 nmwavelengths, or about 25 meV to 50 meV in energy units. VCSELs typicallyhave adjacent resonance's separated by 50 nm to 100 nm, thus only oneresonance may lie within the emission spectrum. As a consequence, thewavelength of this resonance is the wavelength at which the VCSEL maylase. The VCSEL threshold is lowest when the peak of the gain spectrum(gain peak) coincides with the cavity resonance. To complicate matters,the gain spectrum does not necessarily coincide with the emissionspectrum. However, it is generally difficult to measure the gainspectrum experimentally; thus, in the literature, “gain offset” refersto the difference in wavelength or energy between the cavity resonanceand the peak of the emission spectrum (peak transition wavelength(energy)). Use of the term “gain offset” herein is the same. Thus, theterm “gain offset” as used herein provides a direct relation between thecalculated peak transition wavelength (energy) and the VCSEL lasingwavelength (photon energy).

[0162] Gain offset is used to reduce temperature sensitivity of VCSELoperating currents. The peak transition wavelength (and peak of the gainspectrum) increase with temperature at a rate of about 3 Å/K, while thecavity resonance wavelength also increases but, at about 0.6 Å/K. Thus,if the resonance and gain peak coincide at one temperature, they willnot at other temperatures. If the cavity resonance is tuned to a longerwavelength than the gain peak at room temperature, then a small increasein temperature will bring them closer to coincidence. This helps tocompensate for the reduction in magnitude of the gain spectrum whichalso accompanies the rise in temperature. Depending on the amount ofdetuning and shape of the gain spectrum, a slight rise in temperaturemay result in the operating current (for a given power output) rising,lowering, or not changing at all. For Resonant cavity LEDs, the effectsof “gain offset” are similar even though there is no actual net gain andthe emission is not lasing emission.

[0163] Young et al., in an article entitled “High-PowerTemperature-Insensitive Gain-Offset InGaAs/GaAs Vertical-CavitySurface-Emitting Lasers,” IEEE Photonics Technology Letters, pp.129-132, February 1993, reports optimum temperature insensitivity in aVCSEL emitting at 997 nm. The photoluminescence emission peak wavelengthwas at 972 nm, an offset from the cavity mode of 25 nm (32 meV).Likewise, Catchmark et al., in an article entitled “High Temperature CWoperation of GaAs/AlGaAs High Barrier Gain Offset VCSELs,” ElectronicsLetters, vol. 30 no. 25, pp. 2136-2138 (Dec. 8, 1994), reports a VCSELemitting at 873.5 nm which has a gain offset of ˜35 nm (59 meV). A moreoptimized VCSEL has a gain offset of 20 nm (34 meV). The active regionsof these VCSELs had five 95) quantum wells, as compared to the moretypical one (1) to three (3) quantum wells used in VCSELs Thus, theoverall gain is higher, which allows for a higher gain offset.

[0164] It is to be appreciated that for some forms of edge-emitting(in-plane) lasers, it is possible to use gain offset. The necessarycondition for the use of gain offset is that the laser cavity resonanceswhich may produce lasing are spaced sufficiently far apart that adjacentresonances lie in regions of the gain spectrum having significantlydifferent gain. In-plane lasers satisfying this requirement includedistributed feedback (DFB) lasers and phase-shifted DBF lasers. Suchlasers may utilize gain offset as discussed above.

Substrate Orientation

[0165] Fisher et al. in articles which have appeared in Appl. Phys.Lett. 48, 1223 (1986) and J. Appl. Phys. 61, 1023 (1987) studied theeffectiveness of substrate tilt in reducing dislocation density. Thesearticles are hereby incorporated by reference. In their sturdy, threedifferent substrates were utilized. (100), 4° off toward (011) and 4°off toward (001). It was found that the epilayer on the exact (100)contains massive dislocation networks which thread from the GaAsinterface to the surface. The epilayer thickness was about 2 mm. Thedislocation density near the surface is as high as 10¹⁰ cm⁻². Inaddition, to threading dislocations, a large number of stacking faultsare present along (111).

[0166] The epilayer on the substrate misotiented 4° toward (011) hasfewer dislocations that the exact (100) sample. The dislocations arealso confined to a region approximately 1 μm thick from the GaAsinterface. The dislocation density near the film surface isapproximately tow to three orders lower than that of the exact (100)sample. The tilt toward (001) reduces the surface dislocation densityeven further. The further reduction in this case is due to steps runningin both (011) and (011) directions, which provide a greater number ofsources (steps) for the generation of misfits to accommodate the latticemismatch.

[0167] Anan et al. in an article entitled “Critical Layer Thickness on(111B-Oriented InGaAs Heteroepitaxy,” Applied Physics Letters, vol. 60,pp. 3159-3161, 1992, elucidate the increase in critical thickness gainedby using substrates having (111)B orientation and tilted 2° toward(011). An increase by a factor of 2 over the critical thicknesses forlayers grown on (100) substrates is obtained experimentally andtheoretically based on the methodology introduced by Matthews andBlakeslee The numerical coefficients in equations (1) and (2) of thisapplication are based upon material parameters including the angles atwhich dislocations tend to propagate, the elastic coefficients of thematerials, and other properties. They were considered in equations (1)and (2) for simplicity. These values may differ for different substrateorientations, and in fact yield significantly larger predicted criticalthicknesses for the (111) substrate orientation than for the (100)oriented substrates. Experimental data presented in the article alsoshows that growth at lower temperatures allows thicker layers to begrown before the onset of relaxation by dislocations. Although theexperimental results are obtained via growth by MBE, the theoreticalpredictions are equally valid for any growth technique. It is importantto note that Anan et al., monitored the dislocation formation in situduring the growth. Thus, the strain relaxation was observed without thepresence of an overlayer. Thus, the critical thickness measured shouldbe a factor of 2 smaller than for strained layers which have a thickover layer of unstrained material, as previously discussed.

[0168] Anan et al., have shown that the critical thicknesses, based onthe methodology of Matthews and Blakeslee, has a significant dependenceupon substrate orientation. In particular, for the (111) substrateorientation, the critical thickness is calculated to be larger than for(100) substrates, by a factor of approximately two. The main effect wasfound to be the angular relationship between the growth plane and thepreferred direction of dislocation propagation for zinc-blende crystals.The authors make no mention of lasers or of the advantages of extendingwavelength. This article is hereby incorporated by reference.

[0169] Thus, the critical thickness attainable on (111) oriented GaAssubstrates is twice the CT which solves equation (1) and the presentinvention contemplates its use for the achievement of lasers grown onGaAs substrates and which emit light at 1.3 μm or longer wavelengths.Since InP has the same zinc-blende structure as GaAs, similar advantagesmay be expected for extending the emission wavelength for materialsgrown on InP substrates. Thus, the critical thickness attainable on(111) oriented InP substrates is twice the CT which solves equation (2)and the present invention contemplates its use for the achievement oflasers grown on InP substrates and which emit at 2.5 μm or longerwavelengths.

[0170] The effect of using (111) substrates rather than (100) may beseen in FIGS. 1a, 2a, and 7. The change of substrate orientation (byitself) allows pseudomorphic growth at up to two times the CT, ratherthan one times the CT. If combined with other techniques, use of nominal(111) orientations may allow pseudomorphic growth to well above twotimes the CT.

[0171] A by-product of the use of nominal substrate orientations such asthe (111) orientation is that the strain may induce electric fields, viathe piezoelectric effect. For growth on nominal or slightly tilted (100)substrates this does not occur. The electric fields actually cause adecrease in peak transition energy of the strained quantum wells andthus may be used to further increase the peak transition wavelength.Laurich et al., in an article entitled “Optical Properties of (100) and(111) Oriented GaInAs/GaAs Strained-Layer Superlattices,” Physics ReviewLetters. vol. 62, pp. 649-652, 1989 reports a 20 meV shift toward longerwavelengths in structures grown on (111)B substrates compared to thosegrown on (100) substrates. The In content was lower and the layerthicknessess were thicker than for the structures disclosed in thepresent invention. Thus, the structures described herein may exhibitmuch larger electric filed effects. A 20 meV shift is nearly the same asthe 25 meV shifts for gain offset and/or temperature illustrated inFIGS. 2a and 2 b, and is not illustrated in order to minimize confusionin FIG. 2a. The 20 meV shift can be considered similar to the reductionin peak transition energy due to a 40° C. increase in temperature. Thecurve translates downward by 20 meV.

[0172] Turning now to FIG. 11, there is shown a graph of peak transitionenergy and peak transition wavelength v. lattice constant for a varietyof binary and ternary group III-V semiconductor compounds. The binarycompounds are represented by open circles centered on the appropriatecoordinates. Ternary compounds are illustrated by by lines between therespective binaries. For example, the line between GaAs and In ASrepresents the ternary compound (alloy) InGaAs. Solid lines indicatedirect bandgap materials; dashed lines are for indirect bandgapmaterials. The transition energies and wavelengths are for the variouscompounds in the bulk and unstrined condition. The peak transistionenergies of strained quantum wells must additionally account for theeffects of strain and qunatum confinement. This graph is similar to maythat are presented in the semiconductor art. Amoung the variouspresentations of this data, some discrepencies exist in the finerdetails. However, the general behavior of the data is comparable in motpresentations. The purpose here is to illustrate general properties ofmaterials rather than to use FIG. 11 as a means for precisedetermination.

[0173]FIG. 11 shows two significant similarities between the ternaryalloys InGaAs and GaAsSb (lines from GaAs to InAs and From GaAs toGaAs). These similarities are:

[0174] (1) the lines run nearly parallel in the regions close to GaAs onthe graph, indicating that similar increases in lattice constant byadding In or Sb to nominal GaAs produce similar decreases in peaktransistion energy; and

[0175] (2) InAs and GaSb have nearly the same lattice constant,indicating that incorporating equal concentrations of In or Sb intonormal GaAs produce nearly the same lattice mismatch or strain.

[0176] The result of these similarites means that from a materialsstandpoint, In and Sb may be “interchanged” nearly equally. For example,the alloys In_(0.5)Ga_(0.5)As and In_(0.4)Ga_(0.6)As_(0.9)Sb_(0.1) areexpected to be roughly equivalent in terms of lattice constant and inpeak transition energy. Since the InGaAs line lies below the GaAsSbline, InGaAs has a lower peak transition energy than does GaAsSb havingthe same lattice constant. Therefore, from a strain-bandgap viewpoint,InGaAs is slightly preferred over GaAsSb for long-wavelength emission onGaAs substrates. InGaAs is also preferred due to more chemicalcharacteristics. Despite the preference for InGaAs, the presentinvention includes the use of Sb-containing compounds. In the context ofthis application, statements regarding In_(y)Ga_(1-y)As where y is ≧0.5re ment to include In_(y)Ga_(1-y)As_(1-w)Sb_(w) with (y+w)≧0.5.

[0177] The binary compound TIP has a negative peak transition energy orbandgap. Thus, TIP is classified as a metal. The ternary alloy TlGaP isincluded in FIG. 11 using data from the publication by Asahi et al. Itfollows the general trend that heavier atomic constituents producelarger lattice constants and lower bandgap energies, or lower peaktransition energies. The ternary compound GaAsN is also illustrated inFIG. 11 and appears anomalous. Although GaN has a very small latticeconstant, −4.5 Å, and a very high bandgap energy, greater than 3 eV, theternary GaAsN has an extremely large “bowing parameter” as described byKondow et al. Thus, GaAsN may exhibit very low bandgap energies and iseven metallic for a large range of concentrations.

Combination of Techniques

[0178] Several techniques have been described to extend the emissionwavelength of devices grown on a given substrate Use with the identifiedranges of In and Sb concentrations allows 1.3 μm or longer emissionwavelengths on GaAs substrates. While the preferred substrate is GaAs,the invention contemplates use for any other substrates, for example,InP, GaP, InAs, GaSb or InSb. A summary of the techniques and theireffects are provided below. It should be appreciated that this summaryis merely exemplary and the inventive concept should not be viewed asmerely limited to the examples provided below:

[0179] 1) use of superlattice active layers or quantum wells to decreasepeak transition energy by about 40 meV and allow pseudomorphic thicknessincrease by a factor of about 1.25;

[0180] 2) use of small amounts of nitrogen in the active layers todecrease peak transition energy by about 185 meV for each percent ofnitrogen in the group V portion of the active layer;

[0181] 3) use of strain compensation to allow pseudomorphic thicknessincrease by a factor of about 2.45;

[0182] 4) use of nominal (111) oriented substrates to allowpseudomorphic thickness increase by a factor of about 2.0;

[0183] 5) use of elevated temperature to decrease peak transition energyby about 50 meV per 100K increase in temperature of the active region;and

[0184] 6) Use of gain offset to allow emission energy to be lower thanthe peak transition energy, typically by 25 meV and up to 50 meV ormore.

[0185] It is to be appreciated that all of these techniques are mutuallycompatible, i.e., use of any technique does not significantly diminishthe effectiveness of any other technique. Thus, any or all techniquesmay be used in combination or alone.

Devices

[0186] Turning now to FIG. 8, a device incorporating the teachings ofthe invention will be discussed. Semiconductor structure 50 illustratesthe use of superlattice layers which may be strain compensated as wellas may contain nitrogen. Device 50 is grown on a GaAs substrate 52. Itshould be appreciated that the substrate may also be InP as discussedbelow, but for the purposes of this discussion, the substrate shall beGaAs. Next, a confining layer 54 is grown on substrate 52 by anepitaxial process such is MBE, MOCVD or MOMBE at a temperature of 650°C. for a period of 0.1 hours. This results in confining layer 54 havinga thickness of 1,000 Å. A standard growth rate of 1 μm/hr is utilizedfor the growth of materials in this disclosure, unless otherwise stated.It should be appreciated that the thickness of confining layer 54 mayvary but should be between 6 Å and 50,000 Å. Next, a tensile strainedlayer 56 may be grown by an epitaxial process such as MBE, MOCVD orMOMBE at a temperature of 500° C. for a period of 0.3 minutes. Thisresults in tensile strained layer 56 having a thickness of 50 Å. Itshould be appreciated that the thickness of tensile strained layer maybe between 3 Å and 2,000 Å. Thus, it should be appreciated that if thetechnique of strain compensation is utilized, the tensile strained layer56 will be present. Otherwise, tensile strained layer 56 may not bepresent. When present, tensile strained layer 56 preferably comprisesGaAs_(1-z)P_(z) with 0≦z≦1.0, if substrate 52 comprises GaAs, orIn_(y)Ga_(1-y)As with 0.53≦y≦1.0 if substrate 52 comprises InP. Next, asuperlattice quantum well 58 is grown. It should be appreciated that theparticular growth conditions for the superlattice quantum well 58 willvary with the exact structure of the quantum well 58.

[0187] For providing a general understanding of the application of theinventive concept, we will now describe the construction of a straincompensated superlattice having nitrogen containing layers. It should beappreciated that this description is merely illustrative and should inno way be viewed as limiting the invention to this particular structure.First, a GaAs(N) layer 60 is grown above tensile strained layer 56 by anepitaxial process such as MBE, MOCVD or MOMBE at a temperature of 500°C. for a period of 1 second. This results in GaAs(N) layer 60 having athickness of ˜3 Å, i.e., one monolayer for this example. It should beappreciated that there may be other materials disposed between layers 56and 60. For a detailed discussion of this composition of layer 60,please refer to the Nitrogen Containing Layers section, above. Next, aInAs(N) layer 62 by an epitaxial process such as MBE, MOCVD or MOMBE ata temperature of 500° C. for a period of two seconds. This results inInAs(N) layer 62 having a thickness of ˜6 Å; i.e., two monolayers inthis example. Thus, one period of the superlattice structure 58 isformed. This process is repeated. FIG. 8 illustrates three periods whichterminated in a GaAs(N) layer 60. This is merely illustrative of onesuperlattice structure which closely resembles the superlatticeillustrated in FIG. 4a. For other superlattice structures, please referto FIGS. 4a through 4 d. It is to be appreciated that the possibleinclusion of nitrogen in GaAs(N) layers 60 may not perform a criticalfunction. Rather, it may simplify the process to keep the nitrogenflowing throughout the growth of superlattice quantum well 58 due to theextreme thinness of layers 60 and 62.

[0188] A tensile strained layer 64 may be grown by an epitaxial processsuch as MBE, MOCVD or MOMBE at a temperature of 500° C. for a period of−0.6 minutes. This results in tensile strained layer 64 having athickness of 100 Å. It should be appreciated that the thickness oftensile strained layer may be between 6 Å and 2,000 Å. Thus, it shouldbe appreciated that if the technique of strain compensation is utilized,tensile strain in layer 64 will be present. Otherwise, tensile strain inlayer 64 may not be present. Layers 56 and 64 may be of the samematerial or may be constructed of different semiconductor material orthey may have the same basic composition but be of opposite conductivitytypes. When present, tensile strained layer 64 preferably comprisesGaAs_(1-z)P_(z) with 0≦z≦1.0, if substrate 52 comprises GaAs, orIn_(y)Ga_(1-y)As with 0.53≦y≦1.0 if substrate 52 comprises InP.

[0189] In this example, a multiple quantum well structure is illustratedas may be seen from second quantum well 66. It should be appreciatedthat quantum well 66 need not be present and that a functional devicecontemplated by the invention may have only one quantum well. Theadvantage of having multiple quantum wells is that for a givenelectron-hole density, the optical gain is increased. For convenience,quantum well 66 is constructed in a similar manner as quantum well 58.It should be appreciated that it is contemplated by the invention thatquantum well 66 may be different than quantum well 58.

[0190] Next, a tensile strained layer 68 may be grown by an epitaxialprocess such as MBE, MOCVD or MOMBE at a temperature of 500° C. for aperiod of ˜0.3 minutes. This results in tensile strained layer 68 havinga thickness of 50 Å. It should be appreciated that the thickness oftensile strained layer may be between 3 Å and 2,000 Å. It should beappreciated that if the technique of strain compensation is utilized,the presence of tensile strained layer 68 is preferred. Alternatively,tensile strained layer 68 may not be present. Layers 56, 64 and 68 maybe of the same material or may be constructed of different semiconductormaterial or they may have the same basic composition but be ofalternating conductivity types. When present, tensile strained layers56, 64, and/or 68 preferably comprise GaAs_(1-z)P_(z) with 0≦z≦1.0, ifsubstrate 52 comprises GaAs, or In_(y)Ga_(1-y)As with 0.53≦y≦1.0 ifsubstrate 52 comprises InP.

[0191] Finally, a confining layer 70 is grown by an epitaxial processsuch as MBE, MOCVD or MOMBE on tensile strained layer 68 at atemperature of 650° C. for a period of 0.1 hours. This results inconfining layer 70 having a thickness of 1,000 Å. It should beappreciated that the thickness of confining layer 70 may vary but shouldbe between 6 Å and 50,000 Å. Layers 54 and 70 may be of the samematerial or may be constructed of different semiconductor material orthey may have the same basic composition but be of opposite conductivitytypes.

[0192] As stated above, FIG. 8 is provided for a general understandingof the application of the inventive concept. For a discussion of othersuperlattice structures, strain compensation issues, and the use ofnitrogen, please refer to the respective sections, above. It is to beappreciated that the structure illustrated in FIG. 8 may be incorporatedinto a more complex device structure and that the device may incorporategain offset and/or elevated operating temperature in addition to any orall the measures illustrated in FIG. 8. Furthermore, in addition to anyor all the above, growth of surface 53 of substrate 52 may have anyorientation, for example, (001), 4° off the (001) orientation, (011),(111), 2° off the (111) orientation, (311), or any other orientation.

[0193] Turning now to FIG. 9a, a cross section of a VCSEL whichincorporates the quantum wells of FIGS. 1, 2a, 7 and/or 8 isillustrated. For clarity, like elements have been provided with likereference numeral except that a prime has been added to each referencenumeral where there is a slight difference in the particular element inthis embodiment. The following discussion will focus on the differencesbetween the elements of this embodiment and that of the preferredembodiment.

[0194] Light emitting device 100 which is preferably a vertical cavitysurface emitting laser (VCSEL), but it may also be a resonant-cavitylight emitting diode (RCLED). Device 100 may be grown on substrate 52′.Bottom mirror 102 comprises high-index layers 104 and low-index layers106. On bottom mirror 102 is grown bottom spacer 108, active region 110,top spacer 112, and a layer forming a lens and/or aperture 114. Fordetails on specific lenses and/or apertures and their formation, pleaserefer to is U.S. App. Ser. No. 08/547,165, entitled “Conductive Elementwith Lateral Oxidation Barrier,” filed Dec. 18, 1995; U.S. App. Ser. No.08/659,942, entitled “Light Emitting Device Having an Electrical ContactThrough a Layer containing Oxidized Material,” filed Jun. 7, 1996; U.S.App. Ser. No. 08/686,489 entitled “Lens Comprising at Least One OxidizedLayer and Method for Forming Same,” filed Jul. 25, 1996; and U.S. App.Ser. No. 08/699,697 entitled “Aperture comprising an Oxidized Region anda Semiconductor Material,” filed Aug. 19, 1996. These applications arehereby incorporated by reference.

[0195] Lens and/or aperture 114 has outer segments 115 which do notconduct current and may be oxidized and an inner channel which conductscurrent and may be non-oxidized. On top of lens and/or aperture 114 maybe a top mirror 116 comprising low-index layers 118 and high-indexlayers 120. Bottom mirror 102 may preferably comprise alternatingsemiconductor layers such as GaAs or AlGaAs for layers 104 and AlAs orAlGaAs for layers 106. If substrate 52′ comprises InP, bottom mirror 102may preferably comprise alternating semiconductor and oxidized layers,such as InGaAs or InP for layers 104 and an oxide for layers 106. Topmirror 116 may comprise similar materials as bottom mirror 102, or mayalternatively comprise dielectric materials. Finally, top and bottomelectrical contacts 122, 124 are disposed on respective surfaces of topmirror 116 and substrate 52′. As illustrated, top mirror 116 should beconductive to electrical current. Alternatively, top mirror 116 may benon-conductive, in which case electrical contact 122 should be disposedbelow top mirror 116. Additionally, optional isolation regions 125 areprovided which may be formed by ion implantation. Regions 125 areprovided for electrical isolation between VCSELs. In operation of lightemitting device 100, a light beam 127 emits preferably out through topmirror 116. Alternatively, light beam 127 may emit through substrate52′.

[0196] Turning now to FIG. 9b, an exploded view of active region 110 isillustrated. This particular quantum well structure illustrates multiplequantum wells. It should be appreciated that active region 110 may haveonly one quantum well. The advantage of having multiple quantum wells isthat for a given electron-hole density, the optical gain is increased.For convenience, quantum well 126 is constructed in a similar manner asquantum well 128. It should be appreciated that within invention,quantum well 126 may be different than quantum well 128. Barriers orconfining layers 54′ and 70′ are disposed on either side of quantum well126, 128. It should be appreciated that the quantum wells 126, 128 areconstructed as described by the numerous techniques or combinationsthereof. For example, quantum wells 126, 128 may resemble quantum wells58, 66 or may not incorporate all of the techniques illustrated in FIG.8. It should be appreciated that these techniques are utilized incombination with high In concentrations to reduce the peak transitionenergy of a device, having a GaAs substrate, to allow for an emissionwavelength of 1.3 μm. Thus, the quantum wells 126, 128 may be: (1)superlattice structures as discussed in the above superlattice section;(2) may be strain compensated as discussed above; (3) may incorporatenitrogen in the active layer as discussed above; (4) may be providedwith a particular orientation as determined by the orientation ofsubstrate 52′ as also discussed above; and/or (5) operated at anelevated or reduced temperature as also discussed above. Furthermore,light emitting device 100 may utilize gain offset as discussed above.For brevity, individual combinations are not discussed. But, it shouldbe appreciated that this application contemplates any combination whichincreases the emission wavelength to 1.3 μm or above for a GaAssubstrate.

[0197] It should be appreciated that merely one example of a VCSELstructure has been described. This description is merely illustrativeand should in no way be viewed as limiting the invention to thisparticular structure. For a description of other VCSEL structures whichare contemplated by the invention, please refer to FIGS. 5a through 5 fof U.S. patent application Ser. No. 08/574,165 by Jewell. Thisapplication further includes other VCSEL structures including but notlimited to ion implanted VCSELs and other forms of VCSELs havingoxide-defined apertures which may be used in conjunction with theinventive teachings of this application and is hereby incorporated byreference.

[0198] Turning now to FIG. 10a, a cross section of an edge emittinglaser or a light emitting diode, also termed in-plane laser orlight-emitting diode, which incorporates the quantum wells of FIGS. 1,2a, 7 and/or 8 is illustrated. A light source 100′ which incorporateselements such as substrate 52′, first cladding layer 130, active region110′, second cladding layer 132, top contact 122′ and bottom contact124′. In response to a current flow, light source 100′ emits a beam oflight, for example light beam 134 as in an edge-emitting laser.Additionally, a current blocking layer 136 may be present which maycomprise a partially oxidized layer. Layer 136 has outer segments 138which do not conduct current and may be oxidized and an inner channel140 which conducts current and may be non-oxidized. For a detaileddiscussion on layer 136, the reader is referred to U.S. patentapplication Ser. No. 08/574,165 by Jewell. Optionally, grating lines 142may extend partially or completely across light source 100′ to formgrating 144. Such gratings on an in-plane laser may form a distributedfeedback laser (DFB) laser or a distributed Bragg reflector (DBR) laser.A further option is phase shift region 146 in which grating lines 142are shifted, typically by one quarter wave, to form a phase shifted DFBlaser.

[0199] Turning now to FIG. 10b, an exploded view of active region 110′is illustrated. As may be seen, this particular quantum well structureillustrates multiple quantum wells. It should be appreciated that activeregion 110′ may have only one quantum well. The advantage of havingmultiple quantum wells is that for a given electron-hole density, theoptical gain is increased. For convenience, quantum well 126′ isconstructed in a similar manner as quantum well 128′. It should beappreciated that within the invention, quantum well 126′ may bedifferent than quantum well 128′. Barriers or confining layers 54′ and70′ are disposed on either side of quantum well 126′, 128′. It should beappreciated that the quantum wells 126′, 128′ are conducted as describedby the numerous techniques or

58, 66 or may

incorporate all of the techniques in

ed in FIG. 8. It should be appreciated that these techniques areutilized in combination with high In concentrations to reduce the peaktransition energy of a device, having a GaAs substrate, to allow for anemission wavelength of 1.3 μm. Thus, the quantum wells 126′, 128′ maybe: (1) superlattice structures as discussed in the above superlatticesection; (2) may be strain compensated as discussed above; (3) mayincorporate nitrogen in the active layer as discussed above; (4) may beprovided with a particular orientation as determined by the orientationof substrate 52′ as also discussed above; and/or (5) operated at anelevated or reduced temperature as also discussed above. Furthermore,certain forms of light emitting device 100 may utilize gain offset asdiscussed above. For brevity, individual combinations are not discussed.But, it should be appreciated that this application contemplates anycombination which increases the emission wavelength to 1.3 μm or abovefor a GaAs substrate.

[0200] It should be appreciated that merely one example of an in-planelaser structure has been described. This description is merelyillustrative and should in no way be viewed as limiting the invention tothis particular structure. Other such in-plane lasers include but arenot limited to distributed feedback (DFB) lasers, phase-shifted DFBlasers, distributed Bragg reflector (DBR) lasers, angle-facet surfaceemitting lasers and grating surface emitting lasers.

Transition Energy/Wavelength for Strained InAs Quantum Wells on InPSubstrates and Extension Thereof Using the Inventive Techniques

[0201] The room temperature (300K) peak transition energy vs. quantumwell thickness is given in FIG. 7 for InAs quantum wells grown on InPsubstrates and clad by lattice-matched InGaAs barriers. The upper curverepresents the nominal peak transition energy, which accounts for bulkbandgap energy, strain effects and quantum confinement energy. Soliddots are the actual data points calculated. The data is in reasonablyclose agreement with experimental data published by Ploog, et al. Theopen circles on the curve are at the thickness CT (as calculated fromequation 2) and at multiples 0.75, 1.5, and 2.0 of the MBCT. To easeinterpretation, various relevant wavelengths are indicated by dashedlines and are labeled

right axis. The CT for InAs on

with overlayer) is 48.5 Å. The dashed line labeled 1.0X CT correspondsto this thickness, and the open circles within it is at a transitionenergy corresponding to an emission wavelength of 2.3 μm. Withinpublished state-of-the-art structures, this represents the maximumpractical emission wavelength for an InAs quantum well grown on InP. Theother curves shown in FIG. 7 will be discussed in context of thewavelength-extending approaches described by the invention. It should beappreciate that the CT discussed with regard to FIG. 7 is determined byequation 2.

Gain Offset

[0202] Detuning the cavity resonance (determined by mirror and spacerlayer thicknesses) to energies 25 meV lower than the quantum welltransition energy is routinely used which allows a 25 meV reduction inemission wavelength. This discussion assumes that 25 meV gain offsetremains valid for the long-wavelength devices proposed. The assumptionis reasonable but could represent an underestimate or an over estimate.Thus a VCSEL which emits at 2.7 μm (0.459 eV) would favorably use aquantum well with a peak transition energy of about 0.484 eV, whichcorresponds to a pea transition wavelength of 2.56 μm. Conversely, anactive material with a peak transition wavelength of 2.56 μm should beabout optimum for a 2.7 μm-emitting VCSEL. This technique is mainlyapplicable to VCSELs but is also applicable to some forms of in-planedevices as discussed above. The second highest (solid) curve in FIG. 7is located 25 meV below the nominal curve and it represents the VCSELemission wavelengths attainable with the conventional InAs quantumwells. Using 25 meV gain offset alone, the maximum VCSEL emission iscalculated to be about 2.43 μm.

InAsSb Quantum Wells

[0203] Calculations indicate that the transition energy may be, furtherdecreased by replacing some of the As with Sb to for InAsSb quantumwells. This produces a larger strain and thinner MBCT. InAs has alattice mismatch of 3.3% from InP. Increasing mismatch to 4.6% shoulddecrease the transition energy by at least 100 meV in an InAsSb quantumwell with about 20% Sb. A curve (third highest curve) is illustrated inFIG. 7 at a level 100 meV below the nominal curve. Unfortunately, the CTis calculated to be 29 Å for such a quantum well. Its position on thecurve is indicated by the large open circle. The emission is still wellabove the 2.7μ line. However, with a 25 meV gain offset (lowest curve inFIG. 1), the attainable VCSEL emission is very nearly 2.7 μm. Increasingthe strain much further will greatly decrease the MBCT and the quantumconfinement energy will increase greatly, thereby producing an overallincrease in the transition energy.

InAsN Quantum Wells

[0204] For GaAsN, a concentration of only 1% N decreases the bandgapenergy by 185 meV. Thus it is entirely reasonable to expect a Nconcentration of <0.6% to produce a 100 meV drop in energy, resulting inthe third highest curve of FIG. 7. In this case however, the addition ofN increases the critical thinknessess. Different substrate orientationsmay further increase the thickness to which pseudomorphic structures maybe grown on InP.

Strain-Compensation

[0205] To see the effectiveness of this strain compensation on InPsubstrates, again refer to FIG. 7 with attention to the open circles onthe various lines. Now rather than being limited to the thickness at 1.0times the CT quantum well, thicknesses up to about twice the CT or evenmore may be considered. The attainable emission wavelength is now nearly2.7 μm on the nominal curve (top curve in FIG. 7). With the gain offsetof 25 meV (second highest curve in FIG. 1), 2.7 μm emission is reachedat slightly over 1.5 times the CT. When combined with other techniquessuch as the addition of N, emission wavelengths well over 3 μm may beattainable and 2.7 μm emission may be achieved “comfortably.”

[0206] It should be appreciated that although surface emitting lasers(SELs) such as VCSELs have been discussed above as the preferredstructure for a light emitting device which utilizes the teachings ofthis invention, other structures such as LEPs, or EELs may also benefitfrom the teachings herein. The basis for selecting VCSELs is that theyhave excellent operating characteristics and gain offset may reduceemission energies by 25 meV or more. Nevertheless, the inventiveteaching is properly regarded as facilitating epitaxial growth ofmismatched materials and is applicable to LEDs, RCLEDs, SELs, EELs, andmore generally to epitaxial light emitting devices for increasing peaktransition wavelength therein.

[0207] While we have focused the discussion of the variation of Inconcentration, it should be appreciated that other group IIIsemiconductor materials may be utilized. For example, the Inconcentration may be reduced if sufficient Sb is introduced. Nominally,each percent of Sb is almost as effective as In at reducing peaktransition energy and it increases the lattice constant by about thesame amount. The inclusion of small amounts of Al or P in concentrationsof less than 15% is also contemplated by the invention. The inclusion ofother elements, e.g., other group elements such as I, II, III, IV, V,VI, VII, VIII, transition, or rare-earth elements in small quantities ofless than 5% is contemplated by the invention. Of course, doping of thesemiconductor materials may be used in conjunction with the teachings ofthis invention without departing from the scope of the claims.

[0208] Although the present invention has been fully described inconjunction with the preferred embodiment thereof with reference to theaccompanying drawings, it is to be understood that various changes andmodifications may be apparent to those skilled in the art. Such changesand modifications are to be understood as included within the scope ofthe present invention as defined by the appended claims, unless theydepart therefrom.

What is claimed:
 1. A light emitting device having at least a substrateand an active region, said light emitting device comprising: saidsubstrate having a substrate lattice constant between 5.63 Å and 5.67 Å;a first strained layer having a lattice constant smaller than saidsubstrate lattice constant and being disposed between said substrate andsaid active region; said active region comprising at least onepseudomorphic light emitting active layer disposed above said substrate,said active layer comprising at least In, Ga and As, said active layercomprises at least two strained layers, and a third layer disposedbetween said two strained layers, said active layer having a thicknessequal to or less than 80 Å; and wherein said light emitting device hasan emission wavelength of at least 1.3 μm.
 2. The device recited inclaim 1, wherein said active layer has a concentration of In and Sb of25% or greater of a semiconductor material in said active layer.
 3. Thedevice recited in claim 1, wherein said active layer has a thicknessless than 2.5 times CT, where: where: CT=(0.4374/f)[In(CT/4)+1], where fis an average lattice mismatch of said active layer normalized to alattice constant of 5.65 Å.
 4. The device recited in claim 1, whereinsaid first strained layer has at least one graded interface.
 5. Thedevice recited in claim 1, wherein said first strained layer has atleast one stepped interface.
 6. The device recited in claim 1, whereinsaid first strained layer has at least one superlattice interface. 7.The device recited in claim 1, wherein said first strained layer has atleast one smoothly graded interface.
 8. The device recited in claim 1,wherein said first strained layer comprises GaAs_(1-z)P_(z) with0.01≦z≦1.0.
 9. The device recited in claim 1, further comprising asecond strained layer disposed above said active region.
 10. The devicerecited in claim 9, wherein said second strained layer comprisesGaAs_(1-z)P_(z) with 0.01≦z≦1.0.
 11. The device recited in claim 1,wherein said substrate comprises GaAs.
 12. The device recited in claim1, wherein said substrate consists essentially of GaAs.
 13. The devicerecited in claim 1, wherein said substrate has an orientation between 0°and 5° off the (001) orientation.
 14. The device recited in claim 1,further comprising: a first conductive layer having a first conductivitytype, said first conductive layer being disposed below said active layerand in electrical communication therewith; a second conductive layerhaving a second conductivity type, said second conductive layer beingdisposed above said active layer and in electrical communicationtherewith; and electrical communication means for providing electricalcurrent to said active layer.
 15. The device recited in claim 14,further comprising a bottom mirror disposed below saidradiation-emitting layer and a top mirror disposed above saidradiation-emitting layer, said top and bottom mirrors defining anoptical cavity having a cavity resonance at a resonance wavelengthcorresponding to a resonance energy; said resonance wavelength in μm, asmeasured in vacuum, being equal to 1.24 divided by said resonanceenergy, in electron volts.
 16. The device recited in claim 15, whereinsaid bottom mirror comprises alternating high-index layers and low-indexlayers.
 17. The device recited in claim 16, wherein said low indexlayers comprise oxidized material.
 18. The device recited in claim 15,wherein said top mirror, comprises alternating low index layers and highindex layers.
 19. The device recited in claim 18, wherein said low indexlayers are selected from the group consisting of: oxidized material,low-index dielectric material, relatively-low-index semiconductormaterial, and any combination thereof.
 20. The device recited in claim19, wherein said high index layers are selected from the groupconsisting of: high-index dielectric material, high-index semiconductormaterial and any combination thereof.
 21. The device recited in claim15, further comprising an aperture disposed between said active regionand said top mirror, said aperture having a first and second region,said first region exhibits high electrical resistance; and second regionand has an electrical resistance lower that said first region. 22 Thedevice recited in claim 1, wherein said active region has a peaktransition wavelength of at least 1.24 μm.
 23. The device recited inclaim 21, wherein said first region is oxidized and said second regionis oxidized less than said first region.
 24. The device recited in claim15, wherein said resonance wavelength exceeds a peak transitionwavelength of said active region by at least 0.010 μm.
 25. The devicerecited in claim 14, further comprising a grating layer disposed abovesaid second conductive layer, said grating layer having grating linesextending at least partially or completely across said active region toform a grating, said grating defining an optical cavity having a cavityresonance at a resonance wavelength corresponding to a resonance energy;said resonance wavelength in μm, as measured in vacuum, being equal to1.24 divided by said resonance energy, in electron volts.
 26. The devicerecited in claim 25, wherein said grating lines are shifted byapproximately least one quarter wave or a multiple thereof to form aphase shift grating.
 27. A light emitting device having at least asubstrate and an active region, said light emitting device comprising:said substrate having a substrate lattice constant between 5.63 Å and5.67 Å; a first strained layer having a lattice constant smaller than asubstrate lattice constant and being disposed between said substrate andsaid active region; said active region comprising at least onepseudomorphic light emitting active layer disposed above said substrate,said active layer comprising at least In, Ga and As; said active layerhaving a concentration of In and Sb of 25% or greater of a semiconductormaterial in said active layer, said active layer having a thicknessgreater than CT and less than 2.5 times CT for a given material, where:CT=(0.4374/f)[In(CT/4)+1],  where f is an average lattice mismatch ofsaid active layer normalized to a lattice constant of 5.65 Å; whereinsaid light emitting device has an emission wavelength of at least 1.3μm.
 28. A light emitting device having at least a substrate and anactive region, said light emitting device comprising: said substratehaving a substrate lattice constant between 5.63 Å and 5.67 Å; a firststrained layer disposed between said substrate and said active region,said first strained layer having a first accumulated strain and a firstcritical accumulated strain associated therewith, said first accumulatedstrain being less than said first critical accumulated strain; saidactive region comprising at least one pseudomorphic light emittingactive layer disposed above said substrate, said active layer comprisingat least In, Ga and As; said active layer having a concentration of Inand Sb greater than 25% of a semiconductor material in said activelayer, said active layer having a second accumulated strain and a secondcritical accumulated strain associated therewith, the algebraic sum ofsaid first and second accumulated strains being less than said secondcritical accumulated strain; wherein an algebraic sum said first andsecond critical accumulated strains for a given material equals a strainof said material multiplied by CT for a given material, where:CT=(0.4374/f)[In(CT/4)+1],  where f is an average lattice mismatch ofsaid active layer normalized to a lattice constant of 5.65 Å; whereinsaid light emitting device has an emission wavelength of at least 1.3μm.
 29. The device recited in claim 28, further comprising: a firstconductive layer having a first conductivity type, said first conductivelayer disposed below said active layer and in electrical communicationwith said active layer; a second conductive layer having a secondconductivity type, said second conductive layer being disposed abovesaid active layer and in electrical communication therewith; andelectrical communication means for providing electrical current to saidactive layer.
 30. The device recited in claim 29, further comprising abottom mirror disposed below said radiation-emitting layer and a topmirror disposed above said radiation-emitting layer, said top and bottommirrors defining an optical cavity having a cavity resonance at aresonance wavelength corresponding to a resonance energy; said resonancewavelength in Elm, as measured in vacuum, being equal to 1.24 divided bysaid resonance energy, in electron volts.
 31. The device recited inclaim 29, wherein said bottom mirror comprises alternating high-indexlayers and low-index layers.
 32. The device recited in claim 31, whereinsaid low index layers comprise oxidized material.
 33. The device recitedin claim 30, wherein said top mirror, comprises alternating low indexlayers and high index layers.
 34. The device recited in claim 33,wherein said low index layers are selected from the group consisting of:oxidized material, low-index dielectric material, relatively-low-indexsemiconductor material, and any combination thereof.
 35. The devicerecited in claim 34, wherein said high index layers are selected fromthe group consisting of: high-index dielectric material, high-indexsemiconductor material and any combination thereof.
 36. The devicerecited in claim 30, further comprising an aperture disposed betweensaid active region and said top mirror, said aperture having a first andsecond region.
 37. The device recited in claim 36, wherein said firstregion exhibits high electrical resistance; and second region and has anelectrical resistance lower that said first region.
 38. The devicerecited in claim 36, wherein said first region is oxidized and saidsecond region is oxidized less than said first region.
 39. The devicerecited in claim 30, wherein said active region has a peak transitionwavelength of at least 1.24 μm and said resonance wavelength exceeds apeak transition wavelength of said active region by at least 0.010 μm.40. The device recited in claim 29, further comprising a grating layerdisposed above said second conductive layer, said grating layer havinggrating lines extending at least partially or completely across saidactive region to form a grating, said grating defining an optical cavityhaving a cavity resonance at a resonance wavelength corresponding to aresonance energy; said resonance wavelength in μm, as measured invacuum, being equal to 1.24 divided by said resonance energy, inelectron volts.
 41. The device recited in claim 29, wherein said gratinglines are shifted by approximately least one quarter wave or a multiplethereof to form a phase shift grating.
 42. A light emitting devicehaving at least a substrate and an active region, said light emittingdevice comprising: said substrate comprising having a substrate latticeconstant between 5.63 Å and 5.67 Å; said active region comprising atleast one pseudomorphic light emitting active layer disposed above saidsubstrate, said active layer comprising at least In, Ga, As and N, saidactive layer having a thickness equal to or less than a respective CT,where: CT=(0.4374/f)[In(CT/4)+1],  where f is an average latticemismatch of said active layer normalized to a lattice constant of 5.65Å; wherein said active layer comprises at least two strained layers, anda third layer disposed between said two strained layers, forming asuperlattice having a nitrogen content of at least 0.01% of a group Vsemiconductor material in said active region; and wherein said lightemitting device has an emission wavelength of at least 1.3 μm.
 43. Thedevice recited in claim 42, wherein said active region has a peaktransition wavelength of at least 1.24 μm.
 44. The device recited inclaim 42, wherein said substrate comprises GaAs.
 45. The device recitedin claim 42, wherein said substrate consists essentially of GaAs. 46.The device recited in claim 42, wherein said substrate has a orientationbetween 0° and 5° off the (001) orientation.
 47. The device recited inclaim 42, wherein said superlattice has an average sum of In and Sbconcentrations of greater than 30% of the type three semiconductormaterial.
 48. The device recited in claim 42, wherein said active layersare integral multiples of atomic monolayers.
 49. The device recited inclaim 42, wherein at least two adjacent superlattice layers differ in atleast one constituent element by at least 15%.
 50. The device recited inclaim 42, wherein said superlattice comprises (InAs)₂(GaAs)₁(InAs)₂(GaAs)₁(InAs)₂, wherein InAs and GaAs comprise at least 90% ofthe (InAs) and (GaAs) layers, respectively.
 51. The device recited inclaim 42, wherein said superlattice comprises (InAs)₃(GaAs)₁ (InAs)₃,wherein InAs and GaAs comprise at least 90% of the (InAs) and (GaAs)layers, respectively.
 52. The device recited in claim 42, wherein saidsuperlattice comprises(InAs)₁(GaAs)₁(InAs)₂(GaAs)₁(InAs)₂(GaAs)₁(InAs)₁, wherein InAs and GaAscomprise at least 90% of the (InAs) and (GaAs) layers, respectively. 53.The device recited in claim 42, wherein said superlattice comprises(InAs)₁(GaAs)₁(InAs)₄(GaAs)₁(InAs)₁, wherein InAs and GaAs comprise atleast 90% of the (InAs) and (GaAs) layers, respectively.
 54. The devicerecited in claim 42, wherein said superlattice comprises(InAs)₂(GaAs)₂(InAs)₄(GaAs)₂(InAs)₂, wherein InAs and GaAs comprise atleast 90% of the (InAs) and (GaAs) layers, respectively.
 55. The devicerecited in claim 42, wherein said active region consists essentially ofIn_(y)Ga_((1-y))As_(1-(w+v))Sb_(w)N_(v), where v≦0.0095, and w+y≧0.30.56. The device recited in claim 42, wherein said active region consistsessentially of In_(y)Ga_((1-y))As_(1-(w+v))Sb_(w)N_(v), where v≦0.0069,and w+y≧0.33.
 57. The device recited in claim 42, wherein said activeregion consists essentially of In_(y)Ga_((1-y))As_(1-(w+v))Sb_(w)N_(v),where v≦0.0047, and w+y≧0.4.
 58. The device recited in claim 42, whereinsaid active region consists essentially ofIn_(y)Ga_((1-y))As_(1-(w+v))Sb_(w)N_(v), where v≦0.0014, and w+y≧0.5.59. The device recited in claim 42, wherein said emission wavelength ofat least 1.3 μm occurs with said active layer at a temperature of 300Kor less.
 60. The device recited in claim 42, wherein said emissionwavelength is 1.3 μm with said active layer at a temperature greaterthan 300K.
 61. The device recited in claim 42, further comprising: afirst conductive layer having a first conductivity type, said firstconductive layer disposed in electrical communication with said activelayer; a second conductive layer having a second conductivity type, saidsecond conductive layer being disposed above said active layer and inelectrical communication therewith; and electrical communication meansfor providing electrical current to said active layer.
 62. The devicerecited in claim 61, further comprising a bottom mirror disposed belowsaid active layer and a top mirror disposed above said active layer,said top and bottom mirrors defining an optical cavity having a cavityresonance at a resonance wavelength corresponding to a resonance energy;said resonance wavelength in μm, as measured in vacuum, being equal to1.24 divided by said resonance energy, in electron volts.
 63. The devicerecited in claim 62, wherein said bottom mirror comprises alternatinghigh-index layers and low-index layers.
 64. The device recited in claim62, wherein said top mirror, comprises alternating low index layers andhigh index layers.
 65. The device recited in claim 62, furthercomprising an aperture disposed between said active region and said topmirror, said aperture having a first and second region, said firstregion exhibits high electrical resistance; and second region and has anelectrical resistance lower that said first region.
 66. The devicerecited in claim 68, wherein said first region is oxidized and saidsecond region is oxidized less than said first region.
 67. The devicerecited in claim 62, wherein said active region has a peak transitionwavelength of at least 1.24 μm and said resonance wavelength exceeds apeak transition wavelength of said active region by at least 0.010 μm.68. The device recited in claim 61, further comprising a grating layerdisposed above said second conductive layer, said grating layer havinggrating lines extending at least partially or completely across saidactive region to form a grating, said grating defining an optical cavityhaving a cavity resonance at a resonance wavelength corresponding to aresonance energy; said resonance wavelength in μm, as measured invacuum, being equal to 1.24 divided by said resonance energy, inelectron volts.
 69. The device recited in claim 68, wherein said gratinglines are shifted by approximately least one quarter wave or a multiplethereof to form a phase shift grating.
 70. The device recited in claim62, wherein said active region consists essentially ofIn_(y)Ga_((1-y))As_(1-(w+y))Sb_(w)N_(v), where v≦0.0095, and w+y≧0.30.71. The device recited in claim 62, wherein said active region consistsessentially of In_(y)Ga_((1-y))As_(1-(w+v))Sb_(w)N_(v), where v≦0.0044,and w+y≧0.33.
 72. The device recited in claim 62, wherein said activeregion consists essentially of In_(y)Ga_((1-y))As_(1-(w+v))Sb_(w)N_(v),where v≦0.0022, and w+y≧0.4.
 73. The device recited in claim 62, whereinsaid active region consists essentially ofIn_(y)Ga_((1-y))As_(1-(w+v))Sb_(w)N_(v), where v≦0.0006, and w+y≧0.5.74. A light emitting device having at least a substrate and an activeregion, said light emitting device comprising: said substrate comprisinghaving a substrate lattice constant between 5.63 Å and 5.67 Å; a firststrained layer disposed between said substrate and said active region,said first strained layer having a first accumulated strain and a firstcritical accumulated strain associated therewith, said first accumulatedstrain being less than said first critical accumulated strain; saidactive region comprising at least one pseudomorphic light emittingactive layer disposed above said substrate, said active layer comprisingat least In, Ga, As and N, said active layer comprising at least twostrained layers, and a third layer disposed between said two strainedlayers, forming a superlattice having a nitrogen content of at least0.01% of a group V semiconductor material in said active region, saidactive layer having a second accumulated strain and a second criticalaccumulated strain associated therewith, the algebraic sum of said firstand second accumulated strains being less than said second criticalaccumulated strain; wherein said first and second critical accumulatedstrain for a given material equal a strain of said material multipliedby CT for a given material, where: CT=(0.4374/f)[In(CT/4)+1],  where fis an average lattice mismatch of said material normalized to a latticeconstant of 5.65 Å; and wherein said light emitting device has anemission wavelength of at least 1.3 μm.
 75. The device recited in claim74, wherein said active region has a peak transition wavelength of atleast 1.24 μm.
 76. The device recited in claim 74, wherein saidsubstrate comprises GaAs.
 77. The device recited in claim 74, whereinsaid substrate consists essentially of GaAs.
 78. The device recited inclaim 74, wherein said substrate has an orientation between 0° and 5°off the (001) orientation.
 79. The device recited in claim 74, whereinsaid superlattice has an average sum of In and Sb concentrations ofgreater than 12.5% of a semiconductor material in said superlattice. 80.The device recited in claim 74, wherein said active layers are integralmultiples of atomic monolayers.
 81. The device recited in claim 74,wherein at least two adjacent superlattice layers differ in at least oneconstituent element by at least 15%.
 82. The device recited in claim 74,wherein said active region consists essentially ofIn_(y)Ga_((1-y))As_(1-(w+v))Sb_(w)N_(v), where v≦0.0095, and w+y≧0.30.83. The device recited in claim 74, wherein said first strained layercomprises GaAs_(1-z)P_(z) with 0.1≦z≦1.0.
 84. The device recited inclaim 74, further comprising a second strain compensating layer disposedabove said active region, said second strain compensating layer having athird accumulated strain and a third critical accumulated strainassociated therewith.
 85. The device recited in claim 84, wherein saidsecond strained layer comprises GaAs_(1-z)P_(z) with 0.1≦z≦1.0.
 86. Thedevice recited in claim 74, wherein said emission wavelength of at least1.3 μm occurs with said active layer at a temperature of 300K or less.87. The device recited in claim 74, wherein said emission wavelength is1.3 μm with said active layer at a temperature greater than 300K. 88.The device recited in claim 74, further comprising: a first conductivelayer having a first conductivity type, said first conductive layerdisposed in electrical communication with said active layer; a secondconductive layer having a second conductivity type, said secondconductive layer being disposed above said active layer and inelectrical communication therewith; and electrical communication meansfor providing electrical current to said active layer.
 89. The devicerecited in claim 88, further comprising a bottom mirror disposed belowsaid active layer and a top mirror disposed above said active layer,said top and bottom mirrors defining an optical cavity having a cavityresonance at a resonance wavelength corresponding to a resonance energy;said resonance wavelength in μm, as measured in vacuum, being equal to1.24 divided by said resonance energy, in electron volts.
 90. The devicerecited in claim 89, wherein said bottom mirror comprises alternatinghigh-index layers and low-index layers.
 91. The device recited in claim90, wherein said low index layers comprise oxidized material.
 92. Thedevice recited in claim 89, wherein said top mirror, comprisesalternating low index layers and high index layers.
 93. The devicerecited in claim 92, wherein said low index layers are selected from thegroup consisting of: oxidized material, low-index dielectric material,relatively-low-index semiconductor material, and any combinationthereof.
 94. The device recited in claim 92, wherein said high indexlayers are selected from the group consisting of: high-index dielectricmaterial, high-index semiconductor material and any combination thereof.95. The device recited in claim 89, further comprising an aperturedisposed between said active region and said top mirror, said aperturehaving a first and second region.
 96. The device recited in claim 95,wherein said first region exhibits high electrical resistance; andsecond region and has an electrical resistance lower that said firstregion.
 97. The device recited in claim 95, wherein said first region isoxidized and said second region is oxidized less than said first region.98. The device recited in claim 89, wherein said active region has apeak transition wavelength of at least 1.24 μm and said resonancewavelength exceeds a peak transition wavelength of said active region byat least 0.010 μm.
 99. The device recited in claim 98, furthercomprising a grating layer disposed above said second conductive layer,said grating layer having grating lines extending at least partially orcompletely across said active region to form a grating, said gratingdefining an optical cavity having a cavity resonance at a resonancewavelength corresponding to a resonance energy; said resonancewavelength in μm, as measured in vacuum, being equal to 1.24 divided bysaid resonance energy, in electron volts.
 100. The device recited inclaim 99, wherein said grating lines are shifted by approximately leastone quarter wave or a multiple thereof to form a phase shift grating.101. The device recited in claim 89, wherein said active region consistsessentially of In_(y)Ga_((1-y))As_(1-(w+v))Sb_(w)N_(v), where v≦0.0033,and w+y≧0.33.
 102. The device recited in claim 89, wherein said activeregion consists essentially of In_(y)Ga_((1-y))As_(1-(w+v))Sb_(w)N_(v),where v≦0.0068, and w+y≧0.4.
 103. A light emitting device having atleast a substrate and an active region, said light emitting devicecomprising: said substrate comprising having a substrate latticeconstant between 5.63 Å and 5.67 Å; a first strained layer having alattice constant smaller than aid substrate lattice constant and beingdisposed between said substrate and said active region; said activeregion comprising at least one pseudomorphic light emitting active layerdisposed above said substrate, said active layer comprising at least In,Ga, As and N, said active layer comprises at least two strained layers,and a third layer disposed between said two strained layers, said activelayer having a nitrogen concentration of at least 0.01% of a group Vsemiconductor material in said active layer, said active layer having athickness equal to or less than 175 Å; and wherein said light emittingdevice has an emission wavelength of at least 1.3 μm.
 104. The devicerecited in claim 103, wherein said active layer has a thickness equal toor less than 144 Å.
 105. The device recited in claim 103, wherein saidsubstrate comprises GaAs.
 106. The device recited in claim 103, whereinsaid substrate consists essentially of GaAs.
 107. The device recited inclaim 103, wherein said substrate has an orientation between 0° and 5°off the (001) orientation.
 108. The device recited in claim 103, whereinsaid superlattice has an average sum of In and Sb concentrations ofgreater than 16.5% of a semiconductor material in said superlattice.109. The device recited in claim 103, wherein said active regionconsists essentially of In_(y)Ga_((1-y))As_(1-(w+v))Sb_(w)N_(v), wherev≦0.0095, and w+y≧0.33.
 110. The device recited in claim 103, whereinsaid active region consists essentially ofIn_(y)Ga_((1-y))As_(1-(w+v))Sb_(w)N_(v), where v≦0.009, and w+y≧0.33.111. The device recited in claim 103, wherein said active regionconsists essentially of In_(y)Ga_((1-y))As_(1-(w+v))Sb_(w)N_(v), wherev≦0.0068, and w+y≧0.4.
 112. The device recited in claim 103, whereinsaid active region consists essentially ofIn_(y)Ga_((1-y))As_(1-(w+v))Sb_(w)N_(v), where v≦0.0035, and w+y≧0.5.113. The device recited in claim 103, wherein said first strained layercomprises GaAs_(1-z)P_(z) with 0.01≦z≦1.0.
 114. The device recited inclaim 103, wherein said second strained layer comprises GaAs_(1-z)P_(z)with 0.01≦z≦1.0.
 115. The device recited in claim 103, wherein saidemission wavelength of at least 1.3 μm occurs with said active layer ata temperature of 300K or less.
 116. The device recited in claim 103,wherein said emission wavelength is 1.3 μm with said active layer at atemperature greater than 300K.
 117. The device recited in claim 103,further comprising: a first conductive layer having a first conductivitytype, said first conductive layer disposed in electrical communicationwith said active layer; a second conductive layer having a secondconductivity type, said second conductive layer being disposed abovesaid active layer and in electrical communication therewith; andelectrical communication means for providing electrical current to saidactive layer.
 118. The device recited in claim 117, further comprising abottom mirror disposed below said active layer and a top mirror disposedabove said active layer, said top and bottom mirrors defining an opticalcavity having a cavity resonance at a resonance wavelength correspondingto a resonance energy; said resonance wavelength in μm, as measured invacuum, being equal to 1.24 divided by said resonance energy, inelectron volts.
 119. The device recited in claim 118, further comprisingan aperture disposed between said active region and said top mirror,said aperture having a first and second region.
 120. The device recitedin claim 119, wherein said first region exhibits high electricalresistance; and second region and has an electrical resistance lowerthat said first region.
 121. The device recited in claim 119, whereinsaid first region is oxidized and said second region is oxidized lessthan said first region.
 122. The device recited in claim 118, whereinsaid active region has a peak transition wavelength of at least 1.24 μmand said resonance wavelength exceeds a peak transition wavelength ofsaid active region by at least 0.010 μm.
 123. The device recited inclaim 117, further comprising a grating layer disposed above said secondconductive layer, said grating layer, having grating lines extending atleast partially or completely across said active region to form agrating, said grating defining an optical cavity having a cavityresonance at a resonance wavelength corresponding to a resonance energy;said resonance wavelength in μm, as measured in vacuum, being equal to1.24 divided by said resonance energy, in electron volts.
 124. Thedevice recited in claim 123, wherein said grating lines are shifted byapproximately least one quarter wave or a multiple thereof to form aphase shift grating.
 125. A light emitting device having at least asubstrate and an active region, said light emitting device comprising:said substrate comprising having a substrate lattice constant between5.63 Å and 5.67 Å; said substrate comprising having a substrate latticeconstant between 5.63 Å and 5.67 Å; a first strained layer having alattice constant smaller than aid substrate lattice constant and beingdisposed between said substrate and said active region; said activeregion comprising at least one pseudomorphic light emitting active layerdisposed above said substrate, said active layer comprising at least In,Ga, As and N, said active layer comprises at least two strained layers,and a third layer disposed between said two strained layers, said activelayer having a nitrogen concentration of at least 0.01% of a group Vsemiconductor material in said active layer, said active layer having athickness equal to or greater than CT for a given material, where:CT=(0.4374/f)[In(CT/4)+1],  where f is an average lattice mismatch ofsaid active layer normalized to a lattice constant of 5.65 Å; andwherein said light emitting device has an emission wavelength of atleast 1.3 μm.
 126. The device recited in claim 125, wherein said activelayer has a thickness equal to or less than 2.5 times said CT.
 127. Thedevice recited in claim 125, wherein said active layer has a thicknessequal to or less than 2.0 times said CT.
 128. A light emitting devicehaving at least a substrate and an active region, said light emittingdevice comprising: said substrate comprising having a substrate latticeconstant between 5.63 Å and 5.67 Å; a first strained layer disposedbetween said substrate and said active region, said first strained layerhaving a first accumulated strain and a first critical accumulatedstrain associated therewith, said first accumulated strain being lessthan said first critical accumulated strain; said active regioncomprising at least one pseudomorphic light emitting active layerdisposed above said substrate, said active layer comprising at least In,Ga, As and N, said active layer comprises at least two strained layers,and a third layer disposed between said two strained layers, said activelayer having an average sum of In and Sb concentrations in saidsuperlattice at 33% or greater and said nitrogen content of at least0.01% of a group V semiconductor material in said active region, saidactive layer having a second accumulated strain and a second criticalaccumulated strain associated therewith, the algebraic sum of said firstand second accumulated strain being less than said second criticalaccumulated strain; wherein said first and second critical accumulatedstrain for a given material equals a strain of said material multipliedby CT for a given material, where, CT=(0.4374/f)[In(CT/4)+1],  where fis an average lattice mismatch of said active layer normalized to alattice constant of 5.65 Å; and wherein said light emitting device hasan emission wavelength of at least 1.3 μm.
 129. A light emitting devicehaving at least a substrate and an active region, said light emittingdevice comprising: said substrate comprising having a substrate latticeconstant between 5.63 Å and 5.67 Å; a first strained layer having alattice constant smaller than said substrate lattice constant and beingdisposed between said substrate and said active region; said activeregion comprising at least one pseudomorphic light emitting active layerdisposed above said substrate, said active layer comprising at least In,and Ga, said active layer comprising at least two strained layers, and athird layer disposed between said two strained layers, said active layerhaving a second accumulated strain and a second critical accumulatedstrain associated therewith, the algebraic sum of said first and secondaccumulated strains being less than said second critical accumulatedstrain; and wherein said light emitting device has an emissionwavelength of at least 1.3 μm.
 130. The device recited in claim 129,wherein said active layer has a concentration of In and Sb of 25% orgreater of a semiconductor material in said active layer.
 131. Thedevice recited in claim 129, wherein said active layer has a thicknessless than 2.5 times CT, where: where: CT=(0.4374/f)[In(CT/4)+1], whereinf is an average lattice mismatch of said active layer normalized to alattice constant of 5.65 Å.
 132. The device recited in claim 129,wherein said first strained layer comprises GaAs_(1-z)P_(z) with0.1≦z≦1.0.
 133. The device recited in claim 129, further comprising asecond strained layer disposed above said active region.
 134. The devicerecited in claim 133, wherein said second strained layer comprisesGaAs_(1-z)P_(z) with 0.1≦z≦1.0.
 135. The device recited in claim 129,wherein said substrate comprises GaAs.
 136. The device recited in claim129, wherein said substrate consists essentially of GaAs.
 137. Thedevice recited in claim 129, wherein said substrate has an orientationbetween 0° and 5° off the (001) orientation.
 138. The device recited inclaim 129, wherein said active layers are integral multiples of atomicmonolayers.
 139. The device recited in claim 129, wherein at least twoadjacent superlattice layers differ in at least one constituent elementby at least 15%.
 140. The device recited in claim 129, wherein saidsuperlattice comprises (InAs)₂(GaAs)₁(InAs)₂(GaAs)₁(InAs)₂, wherein InAsand GaAs comprise at least 90% of the (InAs) and (GaAs) layers,respectively.
 141. The device recited in claim 129, further comprising:a first conductive layer having a first conductivity type, said firstconductive layer disposed in electrical communication with said activelayer; a second conductive layer having a second conductivity type, saidsecond conductive layer being disposed above said active layer and inelectrical communication therewith; and electrical communication meansfor providing electrical current to said active layer.
 142. The devicerecited in claim 141, further comprising a bottom mirror disposed belowsaid radiation-emitting layer and a top mirror disposed above saidradiation-emitting layer, said top and bottom mirrors defining anoptical cavity having a cavity resonance at a resonance wavelengthcorresponding to a resonance energy; said resonance wavelength in μm, asmeasured in vacuum, being equal to 1.24 divided by said resonanceenergy, in electron volts.
 143. The device recited in claim 142, whereinsaid active region has a peak transition wavelength of at least 1.24 μmand said resonance wavelength exceeds a peak transition wavelength ofsaid active region by at least 0.010 μm.
 144. The device recited inclaim 141, further comprising a grating layer disposed above said secondconductive layer, said grating layer having grating lines extending atleast partially or completely across said active region to form agrating, said grating defining an optical cavity having a cavityresonance at a resonance wavelength corresponding to a resonance energy;said resonance wavelength in μm, as measured in vacuum, being equal to1.24 divided by said resonance energy, in electron volts.
 145. Thedevice recited in claim 144, wherein said grating lines are shifted byapproximately least one quarter wave or a multiple thereof to form aphase shift grating.
 146. A light emitting device having at least asubstrate and an active region, said light emitting device comprising:said substrate comprising having a substrate lattice constant between5.63 Å and 5.67 Å; a first strained layer disposed between saidsubstrate and said active region, said first strained layer having afirst accumulated strain and a first critical accumulated strainassociated therewith, said first accumulated strain being less than saidfirst critical accumulated strain; said active region comprising atleast one pseudomorphic light emitting active layer disposed above saidsubstrate, said active layer comprising at least In, Ga and As, saidactive layer comprises at least two strained layers, and a third layerdisposed between said two strained layers, forming a superlattice havingan average sum of In and Sb concentrations in said superlattice at 25%or greater of a semiconductor material in said active layer, said activelayer having a second accumulated strain and a second criticalaccumulated strain associated therewith, said second accumulated strainbeing less than said second critical accumulated strain; wherein saidfirst and second critical accumulated strain for a given material equalsa strain of said material multiplied by CT for a given material, where:CT=(0.4374/f)[In(CT/4)+1],  where f is an average lattice mismatch ofsaid active layer normalized to a lattice constant of 5.65 Å; andwherein said light emitting device has an emission wavelength of atleast 1.3 μm.
 147. A light emitting device having at least a substrateand an active region, said light emitting device comprising: saidsubstrate comprising having a substrate lattice constant between 5.63 Åand 5.67 Å and having a growth plane which has an orientation within 15°of (111); said active region comprising at least one pseudomorphic lightemitting active layer disposed above said substrate, said active layercomprising at least In, Ga and As, said active layer having a thicknessequal to or less than twice a respective CT, where:CT=(0.4374/f)[In(CT/4)+1],  where f is an average lattice mismatch ofsaid active layer normalized to a lattice constant of 5.65 Å; whereinsaid active layer has an average sum of In and Sb concentrations ofequal to or greater than 25% or greater of a semiconductor material insaid active layer; and wherein said light emitting device has anemission wavelength of at least 1.3 μm.
 148. The device recited in claim147, further comprising: a first conductive layer having a firstconductivity type, said first conductive layer disposed in electricalcommunication with said active layer; a second conductive layer having asecond conductivity type, said second conductive layer being disposedabove said active layer and in electrical communication therewith; andelectrical communication means for providing electrical current to saidactive layer.
 149. The device recited in claim 148, further comprising abottom mirror disposed below said radiation-emitting layer and a topmirror disposed above said radiation-emitting layer, said top and bottommirrors defining an optical cavity having a cavity resonance at aresonance wavelength corresponding to a resonance energy; said resonancewavelength in μm, as measured in vacuum, being equal to 1.24 divided bysaid resonance energy, in electron volts.
 150. The device recited inclaim 149, wherein said bottom mirror comprises alternating high-indexlayers and low-index layers.
 151. The device recited in claim 150,wherein said low index layers comprise oxidized material.
 152. Thedevice recited in claim 149, wherein said top mirror, comprisesalternating low index layers and high index layers.
 153. The devicerecited in claim 152, wherein said low index layers are selected fromthe group consisting of: oxidized material, low-index dielectricmaterial, relatively-low-index semiconductor material, and anycombination thereof.
 154. The device recited in claim 153, wherein saidhigh index layers are selected from the group consisting of: high-indexdielectric material, high-index semiconductor material and anycombination thereof.
 155. The device recited in claim 149, furthercomprising an aperture disposed between said active region and said topmirror, said aperture having a first and second region.
 156. The devicerecited in claim 155, wherein said first region exhibits electricalresistance; and second region and has an electrical resistance lowerthat said first region.
 157. The device recited in claim 155, whereinsaid first region is oxidized and said second region is oxidized lessthan said first region.
 158. The device recited in claim 149, whereinsaid active region has a peak transition wavelength of at least 1.24 μmand said resonance wavelength exceeds a peak transition wavelength ofsaid active region by at least 0.010 μm.
 159. The device recited inclaim 148, further comprising a grating layer disposed above said secondconductive layer, said grating layer having grating lines extending atleast partially or completely across said active region to form agrating, said grating defining an optical cavity having a cavityresonance at a resonance wavelength corresponding to a resonance energy;said resonance wavelength in μm, as measured in vacuum, being equal to1.24 divided by said resonance energy, in electron volts.
 160. Thedevice recited in claim 159, wherein said grating lines are shifted byapproximately least one quarter wave or a multiple thereof to form aphase shift grating.